Port to OpenBSD.

Port to OpenBSD.

See http://lists.gnu.org/archive/html/emacs-devel/2011-07/msg00688.html
and the surrounding thread.
* minibuf.c (read_minibuf_noninteractive): Rewrite to use getchar
rather than fgets, and retry after EINTR.  Otherwise, 'emacs
--batch -f byte-compile-file' fails on OpenBSD if an inactivity
timer goes off.
* s/openbsd.h (BROKEN_SIGIO): Define.
* unexelf.c (unexec) [__OpenBSD__]:
Don't update the .mdebug section of the Alpha COFF symbol table.