Add and document options to allow rpc.lockd and rpc.statd to run in the
foreground.

This allows a separate process to monitor when and how those programs exit.
That process can then restart them if needed.

Submitted by: Alex Burlyga
Reviewed by: bcr, imp
MFC after: 1 week
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D22474