Use EXTENSIONS instead of SKIPIF section

Fix #79100: Wrong FTP error messages

First we need to properly clear the `inbuf`, what is an amendment to
commit d2881adcbc[1].

Then we need to report `php_pollfd_for_ms()` fail

Fix #79100: Wrong FTP error messages

First we need to properly clear the `inbuf`, what is an amendment to
commit d2881adcbc[1].

Then we need to report `php_pollfd_for_ms()` failures right away; just
setting `errno` does not really help, since at least in some cases it
would have been overwritten before we actually could check it. We use
`php_socket_strerror()` to get a proper error message, and define
`ETIMEDOUT` to the proper value on Windows; otherwise we catch the
definition in errno.h, which is not compatible with WinSock. The
proper solution for this issue would likely be to include something
like ext/sockets/windows_common.h.

Finally, we ensure that we only report warnings using `inbuf`, if it is
not empty.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=d2881adcbc9be60de7e7d45a3316b0e11b7eb1e8>.

Closes GH-6718.

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