MFC r280495:

MFC r280495:
Implement a simple OID number garbage collector. Given the increasing
number of dynamically created and destroyed SYSCTLs during runtime it
is very likely that the current new OID number limit of 0x7fffffff can
be reached. Especially if dynamic OID creation and destruction results
from automatic tests. Additional changes:

- Optimize the typical use case by decrementing the next automatic OID
sequence number instead of incrementing it. This saves searching time
when inserting new OIDs into a fresh parent OID node.

- Add simple check for duplicate non-automatic OID numbers.

git-svn-id: svn://svn.freebsd.org/base/stable/10@283171 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f