Use pkexec instead of sudo to add pkg-build group (#19)

There is no sane solution to add current user to pkg-build
group using sudo [1].  Use pkexec for this instead. This works
better if user isn't configured in sudo, but worse on remote
connections like ssh. This trade-off seems reasonable.

Keep sudo configuration for running jobs as pkg-build user,
no point changing that.

[1] http://www.sudo.ws/pipermail/sudo-users/2014-January/005397.html
lpf-gui: refactor: kill some builder_.
manpages: add lpf-gui separate page
lpf-gui: adding icons and .desktop file, README update.
lpf-gui: handle symlinked invocation