vtls: select ssl backend case-insensitive (follow-up)

- Do a case-insensitive comparison of CURL_SSL_BACKEND env as well.

- Change Curl_strcasecompare calls to strcasecompare
  (maps to the former but shorter).

Follow-up to c290b8f.

Bug: c290b8f#commitcomment-24094313

Co-authored-by: Jay Satiro

lib/vauth/digest_sspi.c

@@ -352,8 +352,8 @@ CURLcode Curl_auth_decode_digest_http_message(const char *chlg,
       if(!Curl_auth_digest_get_pair(p, value, content, &p))
         break;
 
-      if(Curl_strcasecompare(value, "stale")
-         && Curl_strcasecompare(content, "true")) {
+      if(strcasecompare(value, "stale") &&
+         strcasecompare(content, "true")) {
         stale = true;
         break;
       }

lib/vtls/vtls.c

@@ -1250,12 +1250,14 @@ static int multissl_init(const struct Curl_ssl *backend)
   if(!env)
     env = CURL_DEFAULT_SSL_BACKEND;
 #endif
-  if(env)
-    for(i = 0; available_backends[i]; i++)
-      if(!strcmp(env, available_backends[i]->info.name)) {
+  if(env) {
+    for(i = 0; available_backends[i]; i++) {
+      if(strcasecompare(env, available_backends[i]->info.name)) {
         Curl_ssl = available_backends[i];
         return 0;
       }
+    }
+  }
 
   /* Fall back to first available backend */
   Curl_ssl = available_backends[0];
@@ -1270,12 +1272,13 @@ CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
   if(Curl_ssl != &Curl_ssl_multi)
     return id == Curl_ssl->info.id ? CURLSSLSET_OK : CURLSSLSET_TOO_LATE;
 
-  for(i = 0; available_backends[i]; i++)
+  for(i = 0; available_backends[i]; i++) {
     if(available_backends[i]->info.id == id ||
-       (name && Curl_strcasecompare(available_backends[i]->info.name, name))) {
+       (name && strcasecompare(available_backends[i]->info.name, name))) {
       multissl_init(available_backends[i]);
       return CURLSSLSET_OK;
     }
+  }
 
   if(avail)
     *avail = (const curl_ssl_backend **)&available_backends;