Allow to specify "cache" and "nocache" as an option for mount_nullfs(8).
Allow to specify "cache" and "nocache" as an option for
mount_nullfs(8).

Tested by:	pho
MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase