Currently, package flagging and unflagging takes place within the
pkg_flag() function. A bool is set to true or false depending on the
action.

Create new pkg_unflag() function with sole purpose of unflagging
and keep pkg_flag() in place. This split will be useful in the
overhaul of the notification system.

Signed-off-by: canyonknight <canyonknight@gmail.com>
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>