MainServer: add a QUERY_FINDFILE command to the protocol

This command will search a storage group on a host for a file and return the
myth url's of any matching file(s) or 'NOT_FOUND' if nothing was found.

The filename can be a regular filename like 'front.jpg' or a regexp like
'front.(jpg|jpeg|png|gif)'.

If allowfallback is '0' only the given host is searched otherwise all hosts
will be searched until a match is found or there is an error.

The format of the command is:
QUERY_FINDFILE <host> <storagegroup> <filename> <useregex (optional)> <allowfallback (optional)>

NOTE: bumps the myth protocol version so all backends and frontends need to be
updated.