Fix cache_dir path handling in MySQLAuth

The path that was given as the option for the cache directory wasn't
properly checked for terminating forward slashes. Due to this, the cache
file was created with the wrong name.

include/maxscale/utils.h

@@ -72,7 +72,7 @@ char* replace_literal(char* haystack,
                       const char* replacement);
 char* replace_quoted(const char** src, const size_t* srcsize, char** dest, size_t* destsize);
 
-void clean_up_pathname(char *path);
+bool clean_up_pathname(char *path);
 
 bool mxs_mkdir_all(const char *path, int mask);