chkservd external check command support (improves checks)

chkservd configuration files allow for a socket connection or checking that a process exists. While this is fine for services which can provide a generic socket response (see: httpd, smtp), it's not ideal for others. Furthermore, simply checking a process exists does not check it is responding to requests.

By adding support for external command responses to chkservd, it will enable support for checking service responses through other means.

For example:

The named configuration in chkservd only checks the process exists, it does nothing more. With support for external commands in chkservd, it could run 'rndc status' and determine if the service is responding.