Setting rc_debug explicitly in /etc/defaults/rc.conf defeats its purpose.

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270391 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add support for /etc/rc.conf.d/<service> subdirectories.  This is
particularly useful for services such as "network" (netif) where each
interface can now have its own separate configuration file.
Add /etc/rc.conf.d to the mtree file so it is always present.

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270392 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f