- autotools: add support for custom prefix.
Usage: `--enable-versioned-symbols=MYPREFIX_`
Catching up with cmake.
- add default versioned prefix for Rustls builds.
- delete `HIDDEN` entry from version script `lib/libcurl.vers.in`.
It referred to symbol names that never existed in libcurl.
- cmake: use `lib/libcurl.vers.in` as a template instead of generating
it from scratch. Adapt `./configure` accordingly, and add comments
to keep hard-wired soname in sync with `lib/Makefile.soname`.
- autotools: delete Schannel and Secure Transport version prefixes.
Windows and macOS don't support the versioned symbols option.
Follow-up to 7b14449790#14378Closes#14818
Allow, at configure time, the production of versioned symbols. The
symbols will look like "CURL_<FLAVOUR>_<VERSION> <SYMBOL>", where
<FLAVOUR> represents the SSL flavour (e.g. OPENSSL, GNUTLS, NSS, ...),
<VERSION> is the major SONAME version and <SYMBOL> is the actual symbol
name. If no SSL library is enabled the symbols will be just
"CURL_<VERSION> <SYMBOL>".
