Add a new file, Makefile.mirrors, which is intended to replace
a 474-line kludge of a shell script to pre-create the directory
hierarchy on ftp-master.

This is not in any way connected to the build, and there is no
intention to do so. This only intent here is to try to make
things a little bit easier for me. But I've probably just made
things worse.

Sponsored by: The FreeBSD Foundation