Allow loading symlinked certificates

When mbedtls_x509_crt_parse_path() checks each object in the supplied path, it only processes regular files. This change makes it also accept a symlink to a file. Fixes Mbed-TLS#3005.

This was observed to be a problem on Fedora/CentOS/RHEL systems, where the ca-bundle in the default location is actually a symlink.

library/x509_crt.c

@@ -1613,7 +1613,7 @@ int mbedtls_x509_crt_parse_path( mbedtls_x509_crt *chain, const char *path )
             goto cleanup;
         }
 
-        if( !S_ISREG( sb.st_mode ) )
+        if( !( S_ISREG( sb.st_mode ) || S_ISLNK( sb.st_mode ) ) )
             continue;
 
         // Ignore parse errors