udev: Use uaccess instead of plugdev (pwr/Solaar#66)

Rules are taken from https://git.lekensteyn.nl/ltunify/. The "VX Nano" receivers
(c51a and c526) have been removed as these are only used for Cordless mice and
are not even enabled in Solaar.

The installer is updated to check for the existence of a group and `mktemp` to
create temporary files. (relying on `$TMPDIR` is a bad idea.) This installer is
too fancy, most users will be fine with `install -m644
rules.d/42-logitech-unify-permissions.rules /etc/udev/rules.d/`. If you are lazy
and want to make `solaar-cli` work over SSH, then this installer will help you.
It won't add you to the `plugdev` group, instead it will edit the rule to use
your user's primary group.

Oh, and it does not change this group to "root" as that is pretty useless given
that root is the default.
Merge pull request #67 from Lekensteyn/updates

udev: Use uaccess instead of plugdev (pwr/Solaar#66)