Add initial support for alternative init systems.

This commit introduces an INIT_SYSTEM variable which will determine the
type of init system to be used on the target system, supporting "rc.d"
at this time.

The pkginstall infrastructure is changed to only install RCD_SCRIPTS if
INIT_SYSTEM is set to "rc.d", and PLIST entries for rc.d scripts are
now handled automatically based on RCD_SCRIPTS.

(jperkin)