Clean up kernel, init, and recovery domains.

Narrow the relabelto rules to a more specific type set
for each domain.

Drop mount permissions from the kernel domain since mounting
occurs after switching to the init domain.  This was likely
a residual of when all processes were left in the kernel domain
on a recovery boot due to the missing setcon statement in the
recovery init.rc.

Be consistent with unlabeled filesystems (i.e. filesystems
without any matching fs_use or genfs_contexts entry) so
that we can also unmount them.

Add comments to note the reason for various rules.

Change-Id: I269a1744ed7bf8c6be899494c5dc97847e5a994d
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>