nixos: transmission improvements

This mostly upgrades transmission, and does some very minor touchups on
AppArmor support.

In particular, there is now no need to ever specify the umask as part of
the settings, as it will be mixed in by default (which is essentially
always what you want). Also, the default configuration is now more
sensible: Downloads are put in /var/lib/transmission/Downloads, and
incomplete files are put in /var/lib/transmission/.incomplete - this
also allows easy use of file syncing probrams, like BitTorrent Sync.

Finally, this unconditionally enables the AppArmor profiles for the
daemon, if AppArmor is enabled - rather than letting the user specify
profile support, it's best to default to supporting profiles for daemons
transparently in all places.

Signed-off-by: Austin Seipp <aseipp@pobox.com>