Tweaks to libarchive's upgrade instructions

I would like to use this file as an example of the FREEBSD-upgrade
convention, see D42302. libarchive is picked somewhat arbitrarily as a
longstanding piece of contrib software in FreeBSD.

- Remove SVN references (HEAD/trunk)
- Mention the vendor/libarchive git branch
- Update link to import instructions
- Remove $FreeBSD$

Reviewed by: mm, imp, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42308

Update information on obtaining libarchive sources and FreeBSD-Xlist

Fix typo s/xz/libarchive/

Reported by: Emil Mikulic (private e-mail)
MFC after: 12 days

Add FREEBSD-Xlist and FREEBSD-upgrade to contrib/libarchive

MFC after: 2 weeks