Fix a few issues with creating VOLUME_LABEL for the
installation ISOs:

 - TYPE, BRANCH, and REVISION are only defined if
   OSRELEASE is not defined, so in situations where
   one might set OSRELEASE for an in-house ISO build,
   VOLUME_LABEL would be empty.

 - makefs(8) limits the volume label to 32 characters,
   which for the powerpc64 case, OSRELEASE expands to
   FreeBSD-11.0-CURRENT-powerpc-powerpc64.  Even with
   removing the prefixing 'FreeBSD-', the string is 30
   characters long, leaving zero room for suffixing the
   type of ISO media (BO for bootonly, CD for cdrom, and
   DVD for dvdrom).

Resolve these by defining VOLUME_LABEL when defining
OSRELEASE if unset.  If OSRELEASE is defined by the
builder, use the OSRELEASE from that definition as the
VOLUME_LABEL.

In addition, for cases where both TARGET and TARGET_ARCH
are used for the VOLUME_LABEL, use TARGET_ARCH if it
differs from TARGET.

There are probably a few sharp edges here yet, but these
problems are going to affect the powerpc/powerpc64 builds
for 10.1-RELEASE, so the immediate concern is fixing the
underlying problem at hand quickly, and less so about the
elegance of the fix.

MFC after:	3 days
X-MFC-10.1:	yes, asap
Sponsored by:	The FreeBSD Foundation
Now that sysctl_root is only called with sysctl lock in shared mode, update
its assertion to require that.

Update comment missed in r273400: sysctl_xlock/unlock -> sysctl_xlock/xunlock

Noted by: jhb