add check-for-remote parameter

[crispyduck00]

As mentioned in another ticket, it would be a useful feature (at least for me) to do nothing and exit instead of doing local snapshots when remotehost not available or remotemaster.
I think to not break things for others this needs to be added via a parameter or a own value in the config file.

Maybe a good place to add it is after the local master check:

		check=`$ZFSGETLVAL ${ZREPTAG}:master $srcfs`
		if [[ "$check" != "yes" ]] ; then
			zrep_errquit $srcfs not master. Cannot sync
		fi

I am doing it for me actually only in zrep_sync, to not create infinite snapshots when remote host is not available or split brain is detected.

[ppbrown]

Note to self: flag on
ZREP_REQUIRE_REMOTE
?