Fixed two return code mixups. CURLE_UNKNOWN_OPTION is saved for when the
option is, yeah, not known. Clarified this in the setopt man page too.

Closes #5993

Since HTTPS is "the new normal", this update changes a lot of man page
examples to use https://example.com instead of the previous "http://..."

Closes #5969

Reported-by: sofaboss on github
Fixes #5942
Closes #5943

Closes #5892

USE_TLS_SRP will be true if *any* selected TLS backend can use SRP

HAVE_OPENSSL_SRP is defined when OpenSSL can use it

HAVE_GNUTLS_SRP is defined when GnuTLS can use it

Clarify in the curl_verison_info docs that CURL_VERSION_TLSAUTH_SRP is
set if at least one of the supported backends offers SRP.

Reported-by: Stefan Strogin
Fixes #5865
Closes #5870

As reported by man(1) when invoked as:

  man --warnings -E UTF-8 -l -Tutf8 -Z <file> >/dev/null

Closes #5846

Follow-up to 6ebe63fac23f38

 const struct curl_easyoption *curl_easy_option_by_name(const char *name);

 const struct curl_easyoption *curl_easy_option_by_id (CURLoption id);

 const struct curl_easyoption *
 curl_easy_option_next(const struct curl_easyoption *prev);

The purpose is to provide detailed enough information to allow for
example libcurl bindings to get option information at run-time about
what easy options that exist and what arguments they expect.

Assisted-by: Jeroen Ooms
Closes #5365

Replace the old fingerprint of the host with a new.

Closes #5685

Closes #5810