builder: Move command line argument processing to separate source file.

This is just code motion.
builder: Remove nbdkit-xz-plugin support (it was disabled anyway).

We can re-add this later if libvirt gets fixed, but for now simplify
the code and concentrate on the one code path.
builder: Wrap program in an exception handler which pretty-prints common exceptions.

This looks like a large change, but is mainly just reindenting
everything into a 'main ()' function.
builder: Remove capital O and lower case L from random root password.

They look too much like 0 and 1.
builder: Really force apt not to ask questions.

This fixes commit b812a0d45992e729a85f10483ae26b82b8e2da90.
sysprep: random seed: Try much harder to create a random seed file.

Since this is essential for security, be much more proactive about
trying to create a random seed file, even for guests which have buggy
installers that don't create this file during installation.
firstboot: Add some debugging to the control script.

For reasons unknown, firstboot does not work on Debian 6 & 7.
https://bugzilla.redhat.com/show_bug.cgi?id=1019388
sysprep: firstboot: Don't set `Created_files flag unless files are created.
firstboot: Ensure firstboot scripts run in command line order.

Previously they ran in effectively random order.
builder: Add maintainer test for new guests.

This is not part of the automated test suite.  It's a manual test run
by the maintainer which must be run on each new guest to ensure that
all the virt-builder features work on the new guest.
Version 1.23.33.