Split s-makepkg into :_s_env and (new) :s-makepkg

This is because the first part of the old :s-makepkg is useful
in other context, e.g. when we need to prepare environment for
for a partial invoke of PKGBUILD (i.e, only run some special
functions from PKGBUILD)
s-get_update: execute :get_update from PKGBUILD

:get_update is part of the :notify_system. It is optional as
not all PKGBUILDs are ported to #theslinux buildsystem.

This function provides a shortcut to invoke :get_update