Allow it to be a variable in the PKGBUILD as well as propagating it through
to the built package and the package database. We leave some backward
compatibility in place by placing the '%FORCE%' option in the database if
the package contains an epoch; this will be used by older versions of pacman
and more or less ignored by versions that use epoch.
Signed-off-by: Dan McGee <dan@archlinux.org>