mirror: Breakout rsync options in arch_mirror
mirror: Revamp Arch mirror script

Inspired by Debian's mirror howto[0] which describes a two-pass
sync to keep a consistent mirror during sync.  Also borrows some
rsync options and conventions from Debian's ftpsync scripts[1].

Still needs some cleanup and stuff, but works fine.

[0] https://www.debian.org/mirror/ftpmirror
[1] http://ftp-master.debian.org/ftpsync.tar.gz