libssh2: replace `access()` with `stat()`

Prefer `stat()` to verify the presence of key files.

This drops the last uses of `access()` in the codebase, which was
reported to cause issues in some cases.

Also add `access()` to the list of banned functions in checksrc.

Ref: https://github.com/curl/curl/pull/13412#issuecomment-2065505415
Ref: https://github.com/curl/curl/pull/13482#issuecomment-2078980522
Ref: #13497
Co-authored-by: Jay Satiro
Closes #13498

lib/vssh/libssh2.c

@@ -1086,6 +1086,7 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
           /* To ponder about: should really the lib be messing about with the
              HOME environment variable etc? */
           char *home = curl_getenv("HOME");
+          struct_stat sbuf;
 
           /* If no private key file is specified, try some common paths. */
           if(home) {
@@ -1093,12 +1094,12 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
             sshc->rsa = aprintf("%s/.ssh/id_rsa", home);
             if(!sshc->rsa)
               out_of_memory = TRUE;
-            else if(access(sshc->rsa, R_OK) != 0) {
+            else if(stat(sshc->rsa, &sbuf)) {
               Curl_safefree(sshc->rsa);
               sshc->rsa = aprintf("%s/.ssh/id_dsa", home);
               if(!sshc->rsa)
                 out_of_memory = TRUE;
-              else if(access(sshc->rsa, R_OK) != 0) {
+              else if(stat(sshc->rsa, &sbuf)) {
                 Curl_safefree(sshc->rsa);
               }
             }
@@ -1107,10 +1108,10 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
           if(!out_of_memory && !sshc->rsa) {
             /* Nothing found; try the current dir. */
             sshc->rsa = strdup("id_rsa");
-            if(sshc->rsa && access(sshc->rsa, R_OK) != 0) {
+            if(sshc->rsa && stat(sshc->rsa, &sbuf)) {
               Curl_safefree(sshc->rsa);
               sshc->rsa = strdup("id_dsa");
-              if(sshc->rsa && access(sshc->rsa, R_OK) != 0) {
+              if(sshc->rsa && stat(sshc->rsa, &sbuf)) {
                 Curl_safefree(sshc->rsa);
                 /* Out of guesses. Set to the empty string to avoid
                  * surprising info messages. */

scripts/checksrc.pl

@@ -720,7 +720,8 @@ sub scanfile {
                     strtok|
                     v?sprintf|
                     (str|_mbs|_tcs|_wcs)n?cat|
-                    LoadLibrary(Ex)?(A|W)?)
+                    LoadLibrary(Ex)?(A|W)?|
+                    access)
                    \s*\(
                  /x) {
             checkwarn("BANNEDFUNC",