- Added a configuration function, 'gethostname()', which calls the system

- Added a configuration function, 'gethostname()', which calls the system
  function of the same name and presents the results as a data expression.
  This function can be used to incorporate the system level hostname of
  the system the DHCP software is operating on in responses or queries (such
  as including a failover partner's hostname in a dhcp message or binding
  scope, or having a DHCP client send any system hostname in the host-name or
  FQDN options by default).  [ISC-Bugs #17351]