pty, readutmp: Fix for FreeBSD 8.0 and OpenBSD 4.6.

pty, readutmp: Fix for FreeBSD 8.0 and OpenBSD 4.6.

* doc/glibc-functions/login_tty.texi: Mention the include file problem
on FreeBSD 8.0 and OpenBSD 4.6.
* lib/pty.in.h: Include <sys/types.h> before <libutil.h>.
* m4/pty_h.m4 (gl_PTY_H): Likewise.
* m4/pty.m4 (gl_FUNC_FORKPTY, gl_FUNC_OPENPTY): Likewise.
* m4/readutmp.m4 (gl_READUTMP): Include <sys/types.h> before <utmp.h>.
Invoke AC_INCLUDES_DEFAULT instead of using the undocumented variable
ac_includes_default.
Reported by Mats Erik Andersson <mats.andersson@gisladisker.se>.