Add -c <command> option to force a specific command

This change adds a -c option to dropbear, to force the session to use a
specific command, in a similar fashion to OpenSSH's ForceCommand
configuration option.

This is useful to provide a simple fixed service over ssh, without
requiring an authorized key file for the per-key forced_command option.

This setting takes precedence over the channel session's provided
command, and the per-key forced_command setting.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>