strparse: string parsing helper functions

Designed to aid converting off from sscanf parsers. sscanf is hard to
use right, easy to mess up and often makes for sloppy error checking.

strparse: string parsing helper functions

Designed to aid converting off from sscanf parsers. sscanf is hard to
use right, easy to mess up and often makes for sloppy error checking.

The new parsers allow more exact and pedandic parsing.

This new set of functions should be possible to use (and extend) and
switch over other libcurl parser code to use going forward.

Adapts the following to use the new functions:

- altsvc.c
- hsts.c
- http_aws_sigv4.c

Bonus: fewer memory copies, fewer stack buffers.

Test: Unit test1664

Docs: docs/internals/STRPARSE.md

Closes #15692

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