arch: x11/xephyr/x11-common

Xorg >=1.16 needs special handling: Configure Xorg.wrap back to
its former behaviour (<1.16) with respect to starting as normal
user.
arch: targets: Most DE and X11-related targets.
arch: chrome-common
arch: kde
arch: gnome

/etc/X11/xinit/xinitrc has a different meaning in Arch, and is
just a default xinit file.
arch: extension: Add desktop file so it can be set as default browser.
arch: audio and tests/30-audio
arch: Do not rely on /etc/os-release anymore

ArchLinux removed it from filesystem package.
arch/bootstrap: Retry downloads.
arch/bootstrap: tar -k fails if files are replaced.

This was actually a bug in tar 1.26, fixed in tar 1.28.

To work around that, we do not extract any of the .* files
in packages (there should still be no conflict.)
arch: Do not test alarm

arch and alarm are actually synonyms.
arch: bootstrap fixups
arch: core/post-common fixups
post-common: Make sure groups actually exist