vtls: make curl_global_sslset thread-safe

.. and update some docs to explain curl_global_* is now thread-safe.

Follow-up to 23af112 which made curl_global_init/cleanup thread-safe.

Closes https://github.com/curl/curl/pull/9016

lib/easy.c

@@ -315,6 +315,23 @@ void curl_global_cleanup(void)
   global_init_unlock();
 }
 
+/*
+ * curl_global_sslset() globally initializes the SSL backend to use.
+ */
+CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
+                              const curl_ssl_backend ***avail)
+{
+  CURLsslset rc;
+
+  global_init_lock();
+
+  rc = Curl_init_sslset_nolock(id, name, avail);
+
+  global_init_unlock();
+
+  return rc;
+}
+
 /*
  * curl_easy_init() is the external interface to alloc, setup and init an
  * easy handle that is returned. If anything goes wrong, NULL is returned.

lib/vtls/vtls.c

@@ -1456,8 +1456,10 @@ static int multissl_setup(const struct Curl_ssl *backend)
   return 0;
 }
 
-CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
-                              const curl_ssl_backend ***avail)
+/* This function is used to select the SSL backend to use. It is called by
+   curl_global_sslset (easy.c) which uses the global init lock. */
+CURLsslset Curl_init_sslset_nolock(curl_sslbackend id, const char *name,
+                                   const curl_ssl_backend ***avail)
 {
   int i;
 
@@ -1486,8 +1488,8 @@ CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
 }
 
 #else /* USE_SSL */
-CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
-                              const curl_ssl_backend ***avail)
+CURLsslset Curl_init_sslset_nolock(curl_sslbackend id, const char *name,
+                                   const curl_ssl_backend ***avail)
 {
   (void)id;
   (void)name;

lib/vtls/vtls.h

@@ -125,6 +125,9 @@ struct curl_slist *Curl_none_engines_list(struct Curl_easy *data);
 bool Curl_none_false_start(void);
 bool Curl_ssl_tls13_ciphersuites(void);
 
+CURLsslset Curl_init_sslset_nolock(curl_sslbackend id, const char *name,
+                                   const curl_ssl_backend ***avail);
+
 #include "openssl.h"        /* OpenSSL versions */
 #include "gtls.h"           /* GnuTLS versions */
 #include "nssg.h"           /* NSS versions */