Whether or not the semaphore code is loaded as a module or built-in, its
sysctl data belongs with the module code.  Move it from kern/init_sysctl.c
to kern/uipc_sem.c

While here, add a new sysctl variable kern.posix.semcnt (current count of
semaphores) to complement the existing kern.posix.semmax (maximum number
of semaphores).

(pgoyette)