Add support to allow an 'include' directive in ctl.conf(5).
This directive expects a directory and will source all configuration files
within that directory ending in '.conf' - similar to syslog.conf(5).
All configuration files sourced in the given directory are processed after
the first level configuration file.
All included configuration files still share the same namespace.
Works with the general syntax and UCL format.
Example ctl.conf(5):
include /path/to/confs
This review is for PR, https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=249239