BUG/MEDIUM: Consistently use 'check' in process_chk

I am not entirely sure that this is a bug, but it seems
to me that it may cause a problem if there agent-check is
configured and there is some kind of error making a connection for it.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Add external check

Add an external check which makes use of an external process to
check the status of a server.