upstream: add a ssh_config RefuseConnection option that, when

upstream: add a ssh_config RefuseConnection option that, when

encountered while processing an active section in a configuration file,
terminates ssh(1) with an error message that contains the argument to the
option.

This may be useful for expressing reminders or warnings in config
files, for example:

Match host foo
       RefuseConnection "foo is deprecated, use splork instead"

ok djg

OpenBSD-Commit-ID: 5b0072fcd08ad3932ab21e27bbaa66b008d44237