daemon: xattr: Remove unused variable.

Revealed by using ./configure --enable-gcc-warnings.
sysprep, builder: Add --timezone option to set timezone of guest.

You can use it like this:

  virt-sysprep --timezone Europe/London ...
  virt-builder --timezone Europe/London ...
builder: docs: Remove confusing reference to timezone.
builder: Document how to change keyboard layout.

This is too complex to implement directly in virt-builder.

Instead we just document how to do it for some common Linux distros
using --run-command, --edit etc.
builder: List --install packages in the same order as on the command line.

Previously the list was reversed.  Note this doesn't have any effect
on how the packages are installed since the guest package manager
doesn't care about the order.
builder: Add --link option for creating symbolic links.

This is useful for configuring systemd services.
builder: Document how to change the language/locale of a new guest.
builder: Document how to set up local mirrors for performance and reliability.
builder: Document that SELinux guests will reboot for autorelabel.