build-sys: fix --disable-all-programs --enable-schedutils

build-sys: fix --disable-all-programs --enable-schedutils

* add UL_ENABLE_ALIAS(NAME, MASTERNAME) to initialize $enable_<name>
  according to MASTERNAME. Note that we have to use $build_<mastername>,
  the $enable_<mastername> is just AC_ARG_ENABLE() stuff only. The
  $build_ is evaluated and modified by our UL_...() functions.

* add enable-schedutils.conf to have build-system regression test for
  this use-case

Addresses: https://github.com/karelzak/util-linux/issues/415
Signed-off-by: Karel Zak <kzak@redhat.com>