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

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",