repack: rewrite the shell script in C (squashing proposal)

This patch is meant to be squashed into bb4335a21441a0
(repack: rewrite the shell script in C), I'll do so when rerolling
the series. For reviewing I'll just send this patch.

* Remove comments, which likely get out of date (authorship is kept in
  git anyway)
* rename get_pack_filenames to get_non_kept_pack_filenames
* catch return value of unlink and fail as the shell version did
* beauty fixes to remove_temporary_files as Junio proposed
* install signal handling after static variables packdir, packtmp are set
* remove adding the empty string to the buffer.
* fix the rollback mechanism (wrong variable name)

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>