builtin-am: support --keep-cr, am.keepcr

builtin-am: support --keep-cr, am.keepcr

Since ad2c928 (git-am: Add command line parameter `--keep-cr` passing it
to git-mailsplit, 2010-02-27), git-am.sh supported the --keep-cr option
and would pass it to git-mailsplit.

Since e80d4cb (git-am: Add am.keepcr and --no-keep-cr to override it,
2010-02-27), git-am.sh supported the am.keepcr config setting, which
controls whether --keep-cr is on by default.

Re-implement the above in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>