rc.d/mountd: Do not require /etc/exports

mountd exits if it cannot read any exports files. Requiring
/etc/exports, however, unconditionally stops mountd from starting on a
server that exports ZFS datasets only (/etc/zfs/exports).

Let sbin/mountd handle requirements itself, instead of hardcoding
/etc/exports, which is also wrong, because different files can be
specified in mountd_flags.

This patch also works with nfsv4_server_enable="YES", by preserving the
behavior of failing to start if there is no "V4:" line in the exports
file, which is missing by default on ZFS-only exports.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Reviewed by:	rmacklem, markj, emaste
Differential Revision:	https://reviews.freebsd.org/D60044

libexec/rc/rc.d/mountd

@@ -13,7 +13,6 @@ desc="Service remote NFS mount requests"
 rcvar="mountd_enable"
 command="/usr/sbin/${name}"
 pidfile="/var/run/${name}.pid"
-required_files="/etc/exports"
 start_precmd="mountd_precmd"
 extra_commands="reload"