Let non-root users use MTP devices

[Chris Marusich]

These small patches enable easier MTP file transfer for devices, such as
Android phones, on GuixSD.

Previously, you had to run a program as root (e.g., "sudo gmtp") to do this,
and the MTP backend for gvfs was not functioning properly (i.e., you could not
use a program like Nautilus to transfer files).  This was because we were
missing some udev rules that the libmtp package provides.  These patches add
those rules and configure libmtp so that the rules will grant the "audio"
group access to the MTP-related device files.  After these patches are
applied, any user in the "audio" group will be able to use tools like "gmtp"
without running them as root, and the MTP backend for gvfs will "just work"
for any user who is in the "audio" group.  I've verified that this works in
GNOME and also Xfce; I was successful in transferring files to an Android
device using drag-and-drop via Nautilus.

Note that in order to use a program like "gmtp," you'll probably need to make
sure no other programs (e.g., "gvfs-mtp-volume-monitor") are using the device
at the same time.  Apparently, when using MTP, only one process can use a
device at a time.  For details, please refer to the documentation in the
libmtp source.

-- 
Chris