Garbage collect ghost sysctl.

Garbage collect ghost sysctl.

p1003_1b.mq_open_max sysctl:

1. Has never worked properly (displayed a wrong value).
2. Didn't allow for its value to be tuned.
3. Is non-standard, so no 3rd party should rely on it. And, thus, noone
   should be harmed by its removal.
   The canonical way to get the maximum open message queue descriptors
   per process is via the sysconf(3) interface.
4. Has now been replaced by kern.mqueue.mq_open_max sysctl which
   addresses 1. and 2.

A stub has been inserted in place of the old sysctl definitions, in order to
reduce unnecessary diffs due to renumbering. This patch does NOT introduce
p1003_1b.unused1.

Partially-discussed-with: swildner@