Allocate buffer for READ BUFFER/WRITE BUFFER commands on demand.

These commands are rare, but consume additional 256KB RAM per LUN.

MFC after:	1 week
Simplify memrw().

MFC after:	10 days
getgrouplist(3) was reimplemented in terms of getgrent_r(3) in r174547, so
this bug is no longer valid.

MFC after:	3 days