Use PNG icons instead of GIF icons

This reduces the icon sizes considerably.
Constify some things in file serving module
Refactor some bits in _create_cache_entry()

After directory list support came in, this was left in a hairy state. Clean
up some of that mess.
Fix dereference of null pointer in template
Plug memleak in symtab_push()
Disable SO_REUSEPORT by default
Hardcode content-type in dirlist when creating the cache entry