nfsd, rpcbind: add -P option to support pidfile path

The -P option, when specified opens a pidfile to a given path.
Both daemons also install a default pid file to the corresponding
locations, that is, /var/run/nfsd.pid and /var/run/rpcbind.pid.

Sponsored by: Juniper Networks, Inc.
MFC after: 7 days
Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D50624