Trap EXIT and use relative cache and db paths

Trapping exit ensures that unmounting occurs in the event of an exit 1
or a normal exit. Pacman 3.0.6 takes cachedir and dbpath to be relative
to it's given root, not absolute.

Signed-off-by: Simo Leone <simo@archlinux.org>
Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>