rrdd plugins should exit on SIGTERM/SIGINT

Otherwise the main loop catches the EINTR and continues running,
and the init script eventually has to send SIGKILL.

Signed-off-by: John Else <john.else@citrix.com>
Add generic plugin cleanup function

Signed-off-by: John Else <john.else@citrix.com>
Remove redundant handling of Sys.Break

It's already handled by the main loop.

Signed-off-by: John Else <john.else@citrix.com>