pager: refactor wait_for_pager() function

pager: refactor wait_for_pager() function

Refactor the wait_for_pager() function. Since 507d7804c0b (pager:
don't use unsafe functions in signal handlers, 2015-09-04) the
wait_for_pager() and wait_for_pager_atexit() callers diverged on more
than they shared.

Let's extract the common code into a new close_pager_fds() helper, and
move the parts unique to the only to callers to those functions.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>