This is a hack.  Unclear if this is really needed or what it does, but
it fixes Ubuntu 11.10 builds for me.

If there is no --use-installed option, then it didn't download all the
packages (only the ones not installed).  But this failed later when it
tried to unpack the packages that hadn't been downloaded.

Thus download all the packages if !use_installed.

Use the Debian ph_init handler to get the list of installed
packages.

use_installed is a global variable (defined in febootstrap_cmdline.mli)
so there's not much point in passing it around to every function that
needs it.  This commit removes the optional argument in favour of just
using the global variable in each package handler.

However we still need a place where we can bail if the --use-installed
flag is used for package handlers which don't support this yet.

Thus add a ph_init function is called after the right package handler
has been detected but before it is used.  This is a convenient place
to put the --use-installed checking and any other initialization that
is required.

Delay calling this until after debian_detect() has been called and
we've decided to use the Debian backend.  Otherwise this breaks
non-dpkg backends.

This fixes commit b8cea4656e5bf1b9e7bb2274ef968ea42b10e0df.

Since the pactree utility only accepts one package name, we must iterate
through the packages rather than specify them all on the command line.
To avoid errors when febootstrap is run on a system with outdated packages,
use ls and awk on the febootstrap package cache directory rather than using
the host's outdated pacman database.

This also adds --use-installed switch (which for now only works for
Debian).

This technique copied from libguestfs/resize/Makefile.am and modified
so that it works with old ocamldep that used to add spaces at the end
of lines.