mountd(8): generate a syslog message when the "V4:" line is missing

Daniel reported that NFSv4 mounts were not working despite having
set "nfsv4_server_enable=YES" in /etc/rc.conf. Mountd was logging a
message that there was no /etc/exports file.
He noted that creating a /etc/exports file with a "V4:" line in it
was needed make NFSv4 mounts work.
At least one "V4:" line in one of the exports(5) file(s) is needed to
make NFSv4 mounts work. This patch fixes mountd.c so that it logs a
message indicting that there is no "V4:" line in any exports(5)
file when NFSv4 mounts are enabled.
To avoid this message being generated erroneously, /etc/rc.d/mountd
is updated to make sure vfs.nfsd.server_max_nfsvers is properly set
before mountd(8) is started.

Reported by: debdrup
PR: 253901
MFC after: 2 weeks