bhyve: Config file syntax needs an include directive

[rosenfeld_grumpf.hope-2000.org]

Summary

bhyve already supports specifying any number of config files on the
command line by using multiple -k options. It would be really useful
if the config file syntax had a mechanism to include other config
files.

The syntax of the new include directive is quite simple:

include <path>

Path can be absolute, or relative to the directory the current config
file is in.