read only /usr, with some writable subdirectories

[Don Hopkins]

If you mount /usr read-only, but want to be able to write to some of the
subdirectories of /usr, you can re-mount them on top of themselves,
read-write. i.e.:

ballast:/up/3.2bin on /usr type nfs (ro,noquota) 
ballast:/usr/unimacs on /usr/unimacs type nfs (rw,noquota) 
ballast:/usr/NeWS on /usr/NeWS type nfs (rw,noquota) 

There is a symbolic link on ballast so that /usr points to /up/3.2bin, so
/usr/unimacs and /usr/NeWS are the same directories as usual, but I can
write to them from the nfs client. 

	-Don