whitespace - document alternative locking under linux
Fix a bug that allows NFS clients to issue READDIR on files.

PR:		kern/178016
Security:	CVE-2013-3266
Security:	FreeBSD-SA-13:05.nfsserver
Add a -Z option which zeroes unused blocks.  It can be combined with -E,
in which case unused blocks are first zeroed and then erased.

Reviewed by:	mckusick
MFC after:	3 weeks
Style nit.