gtls: add return checks and optimize the code

[x2018]

This PR does the following things:

1. Update the description (based on gnutls_global_init()) of gtls_init() and properly check its status in gtls_client_init().

gtls_init() should only be called in curl_global_init()->Curl_ssl_init() and the calling in gtls_client_init() itself is redundant?

2. In gtls_client_init(), check the invaild SSLVERSION at first. Note that this part refactors the duplicate/incompatible checks and removes the current useless local variable sni.
3. Check the return value of gnutls_ocsp_resp_init(). Although the original code is safe because gnutls_ocsp_resp_import() will check the validity of ocsp_resp, it is better to catch the error in time and record the proper message to output log.

[bagder]

Note that clang-tidy still complains:

/home/runner/work/curl/curl/bld/lib/../../lib/hmac.c:171:3: error: Initialized va_list is leaked [clang-analyzer-valist.Unterminated,-warnings-as-errors]
  171 |   return CURLE_OK;
      |   ^
/home/runner/work/curl/curl/bld/lib/../../lib/hmac.c:162:6: note: Assuming 'ctxt' is non-null
  162 |   if(!ctxt)
      |      ^~~~~
/home/runner/work/curl/curl/bld/lib/../../lib/hmac.c:162:3: note: Taking false branch
  162 |   if(!ctxt)
      |   ^
/home/runner/work/curl/curl/bld/lib/../../lib/hmac.c:169:3: note: Initialized va_list
  169 |   Curl_HMAC_final(ctxt, output);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/runner/work/curl/curl/bld/lib/../../lib/hmac.c:171:3: note: Initialized va_list is leaked
  171 |   return CURLE_OK;
      |   ^

[x2018]

It should be a false positive because 066f571 is ok and 5dfaedc just cleanup a piece of outdated note.
Let me find a typo or something similar to try the check again.