Fix rtsold's usage message

• Remove -a from the usage message example dealing with specific interfaces. -a only makes sense when not specifying an interface, such that it's to be run on all interfaces
• Fix the pidfile option (it's -p, not -P)
• Change interfaces to interface to match the manpage

MFC after: 3 days
PR: 173744
Sponsored by: EMC / Isilon Storage Division