Rewrite of d_netfil.c's findfile

This branch rewrites findfile from d_netfil.c so it'll be able to properly report in Linux and the like whether there was a file found in one of its three search paths with a bad MD5, rather than pretending it doesn't exist.

Note: Untested as of writing, but I assume it should be safe for master, since file searching is entirely local. Could be wrong though.