Add much needed descriptions for a number of the IPC related sysctl OIDs.
This information will be very useful for people who are tuning applications
which have a dependence on IPC mechanisms.

The following OIDs were documented:

Message queues:
kern.ipc.msgmax
kern.ipc.msgmni
kern.ipc.msgmnb
kern.ipc.msgtlq
kern.ipc.msgssz
kern.ipc.msgseg

Semaphores:
kern.ipc.semmap
kern.ipc.semmni
kern.ipc.semmns
kern.ipc.semmnu
kern.ipc.semmsl
kern.ipc.semopm
kern.ipc.semume
kern.ipc.semusz
kern.ipc.semvmx
kern.ipc.semaem

Shared memory:
kern.ipc.shmmax
kern.ipc.shmmin
kern.ipc.shmmni
kern.ipc.shmseg
kern.ipc.shmall
kern.ipc.shm_use_phys
kern.ipc.shm_allow_removed
kern.ipc.shmsegs

These new descriptions can be viewed using sysctl -d

PR: kern/65219
Submitted by: Dan Nelson <dnelson at allantgroup dot com> (modified)
No objections: developers@
Descriptions reviewed by: gnn
MFC after: 1 week