Initial import of NetBSD rc system

IMPORTANT: this change has a docs/UPDATING entry!

This patch performs an initial import of the infrastructure and a
subset of the NetBSD set of rc startup and shutdown scripts.  The
"initial" refers to the fact that this is not yet a full switch to the
NetBSD rc system: the MINIX ramdisk rc script, which (typically) runs
as the first thing, is kept as is.  After mounting the root file
system, the ramdisk rc script will start the NetBSD rc infrastructure
by invoking /etc/rc, however.  The regular MINIX startup-and-shutdown
script has been moved from /etc/rc to /etc/rc.minix, and is now
invoked as part of the NetBSD rc infrastructure through a bridge rc
script /etc/rc.d/minixrc.  /etc/rc.minix invokes /usr/etc/rc as before.

Switching over the ramdisk to the NetBSD system and decomposing the
MINIX rc.minix script into smaller components are left to future work.
Also, the current pkgsrc etc/rc.d auto-start functionality is left as
is, even though it should be removed (see the etc/usr/rc comment).

Change-Id: Ia96cae7c426e94b85c67978dc1307dacc4b09fc5

docs/UPDATING

@@ -1,3 +1,9 @@
+20170223:
+	Edit your /etc/rc.conf to set rc_configured to YES.  If you do not,
+	your system will refuse to enter multi-user mode after the next reboot.
+	You can then still make the same edit from single user mode, but that
+	is not as easy.
+
 20170222:
 	Because of naming conflicts, the MINIX 3 service(8) utility has been
 	renamed to "minix-service".  After successfully rebuilding the system