Introduce infrastructure support for SMF.

SMF is the Service Management Facility, the default init system in
Solaris and derivatives since version 10.  This adds "smf" to the list
of supported INIT_SYSTEM types, and makes it the default init system on
platforms where it is available.

Packages can introduce SMF support by providing a manifest file, by
default located in ${FILESDIR}/smf/manifest.xml but manifests under
${WRKSRC} can be used too if the package source includes one.

SMF method scripts are supported too if required, using SMF_METHODS in a
similar manner to RCD_SCRIPTS.

Many parts of the SMF infrastructure are configurable, see mk/smf.mk for
the full details.

(jperkin)