Remove troublesome chars from cachefile names

Add a funtion cache_safe_filename() which replaces possibly bad filename
characters with '_'.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -87,6 +87,7 @@ extern int cgit_read_config(const char *filename, configfn fn);
 extern int cgit_parse_query(char *txt, configfn fn);
 extern struct commitinfo *cgit_parse_commit(struct commit *commit);
 
+extern char *cache_safe_filename(const char *unsafe);
 extern int cache_lock(struct cacheitem *item);
 extern int cache_unlock(struct cacheitem *item);
 extern int cache_cancel_lock(struct cacheitem *item);