curl: add options for safe/no CA bundle search (Windows)

Add `CURL_CA_SEARCH_SAFE` build-time option to enable CA bundle search
in the `curl` tool directory. The lookup method was alread

curl: add options for safe/no CA bundle search (Windows)

Add `CURL_CA_SEARCH_SAFE` build-time option to enable CA bundle search
in the `curl` tool directory. The lookup method was already used to find
`.curlrc` and `_curlrc` (on Windows). On Windows it overrides the unsafe
default `SearchPath()` method.

Enable with:
- cmake: `-DCURL_CA_SEARCH_SAFE=ON`
- autotools: `--enable-ca-search-safe`
- raw: `CPPFLAGS=-DCURL_CA_SEARCH_SAFE`

On Windows, before this patch the whole `PATH` was searched for
a CA bundle. `PATH` may contain unwanted or world-writable locations,
including the current directory. Searching them all is convenient to
pick up any CA bundle, but not secure.

The Muldersoft curl distro implements such CA search via a custom
patch for Windows:
https://github.com/lordmulder/cURL-build-win32/blob/cd652d4792c177c98b08b4309d3cac2b8dbbf9b0/patch/curl_tool_doswin.diff#L50

MSYS2/mingw-w64 distro has also been rolling a patch solving this:
https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-curl/0001-Make-cURL-relocatable.patch
https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-curl/pathtools.c

Also add option to fully disable Windows CA search:
- cmake: `-DCURL_DISABLE_CA_SEARCH=ON`
- autotools: `--disable-ca-search`
- raw: `CPPFLAGS=-DCURL_DISABLE_CA_SEARCH`.

Both options are considered EXPERIMENTAL, with possible incompatible
changes or even (partial) removal in the future, depending on feedback.

An alternative, secure option is to embed the CA bundle into the binary.

Safe search can be extended to other platforms if necessary or useful,
by using `_NSGetExecutablePath()` (macOS),
`/proc/self/exe` (Linux/Cygwin), or `argv[0]`.

Closes #14582

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tidy-up: OS names

Use these words and casing more consistently across text, comments and
one curl tool output:
AIX, ALPN, ANSI, BSD, Cygwin, Darwin, FreeBSD, GitHub, HP-UX, Linux,

tidy-up: OS names

Use these words and casing more consistently across text, comments and
one curl tool output:
AIX, ALPN, ANSI, BSD, Cygwin, Darwin, FreeBSD, GitHub, HP-UX, Linux,
macOS, MS-DOS, MSYS, MinGW, NTLM, POSIX, Solaris, UNIX, Unix, Unicode,
WINE, WebDAV, Win32, winbind, WinIDN, Windows, Windows CE, Winsock.

Mostly OS names and a few more.

Also a couple of other minor text fixups.

Closes #14360

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curl: support embedding a CA bundle

Add the ability to embed a CA bundle into the curl binary. It is used
when no other runtime or build-time option set one.

This helps curl-for

curl: support embedding a CA bundle

Add the ability to embed a CA bundle into the curl binary. It is used
when no other runtime or build-time option set one.

This helps curl-for-win macOS and Linux builds to run standalone, and
also helps Windows builds to avoid picking up the CA bundle from an
arbitrary (possibly world-writable) location (though this behaviour is
not currently disablable).

Usage:
- cmake: `-DCURL_CA_EMBED=/path/to/curl-ca-bundle.crt`
- autotools: `--with-ca-embed=/path/to/curl-ca-bundle.crt`
- Makefile.mk: `CURL_CA_EMBED=/path/to/curl-ca-bundle.crt`

Also add new command-line option `--dump-ca-embed` to dump the embedded
CA bundle to standard output.

Closes #14059

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cmdline-opts: language cleanups

Use imperative mood consistently for the first sentence describing an
option.

"Set this" instead "tell curl to set" or "this sets..."

Pl

cmdline-opts: language cleanups

Use imperative mood consistently for the first sentence describing an
option.

"Set this" instead "tell curl to set" or "this sets..."

Plus some extra cleanups and rephrasing.

Closes #13106

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docs/cmdline: change to .md for cmdline docs

- switch all invidual files documenting command line options into .md,
as the documentation is now markdown-looking.

- made the

docs/cmdline: change to .md for cmdline docs

- switch all invidual files documenting command line options into .md,
as the documentation is now markdown-looking.

- made the parser treat 4-space indents as quotes

- switch to building the curl.1 manpage using the "mainpage.idx" file,
which lists the files to include to generate it, instead of using the
previous page-footer/headers. Also, those files are now also .md
ones, using the same format. I gave them underscore prefixes to make
them sort separately:
_NAME.md, _SYNOPSIS.md, _DESCRIPTION.md, _URL.md, _GLOBBING.md,
_VARIABLES.md, _OUTPUT.md, _PROTOCOLS.md, _PROGRESS.md, _VERSION.md,
_OPTIONS.md, _FILES.md, _ENVIRONMENT.md, _PROXYPREFIX.md,
_EXITCODES.md, _BUGS.md, _AUTHORS.md, _WWW.md, _SEEALSO.md

- updated test cases accordingly

Closes #12751

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