Improve start-up speed on Arch by storing the path to the distribution package

e.g. the selections document now contains entries like this:

  <selection id="package:arch:gnupg:2.0.19-6:x86_64"
             quick-test-file="/var/lib/pacman/local/gnupg-2.0.19-6/desc"
	     ...>

If the quick test file still exists when we run the app then we know we don't
need to check for updates. Previously, we had to scan /var/lib/pacman/local/
looking for an entry with the right prefix.
Moved gobject initialisation to zeroinstall.support.tasks

This means that importing the main "zeroinstall" module no longer loads gobject
(which is slow). Note that if your program depends on zeroinstall as a library and
uses it to download anything (which requires threads) AND you use gobject/GTK then
you must call gobject.threads_init() before using any gobject code. Previously,
"import zeroinstall" would do this for you.

(currently, "import zeroinstall.support.tasks" will call threads_init, but in
future we may delay initialising it even later)

The immediate benefit from this is that 0install-runenv-runenv.py now runs
faster. On my laptop:

Python 2: 56 ms -> 48 ms
Python 3: 93 ms -> 61 ms
Read ArchLinux desc files binary mode

Avoids UnicodeDecodeError if current locale isn't UTF-8.