sed: Fix -i option behavior with 'q' command.

Don't just exit when encountering the 'q' command if we edit file
inplace, and give mf_fgets() a chance to actually handle the
inplace case.

Also add a regression test.

Submitted by: Yuri Pankov <yuripv@yuripv.net>
Approved by: re (kib)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16798