rsh: introduce new option -N disabling shutdown of socket sending path.

This prevents premature disconnection of rsh session with protocol
implementation confused by "end-of-file" condition for standard
input stream. For example, modern Cisco IOS (15.x) versions
can be managed with "rsh -N" cron jobs having /dev/null as stdin.

PR: 205144
Approved by: avg (mentor)
MFC after: 1 week