There is duplicated code in the util.c files in the libalpm and pacman
source code. Split this into a separate file so that it can be shared
via a symlink. This prevents code divergence between the two code bases.

Also, move mbasename and mdirname from pacman/util.c into util-common.c
in preparation for the following patch that uses them to add an extension
to pacsave files.

Signed-off-by: Allan McRae <allan@archlinux.org>