Enforce the maxproc limitation before allocating struct proc, initial
struct thread and kernel stack for the thread. Otherwise, a load
similar to a fork bomb would exhaust KVA and possibly kmem, mostly due
to the struct proc being type-stable.

The nprocs counter is changed from being protected by allproc_lock sx
to be an atomic variable. Note that ddb/db_ps.c:db_ps() use of nprocs
was unsafe before, and is still unsafe, but it seems that the only
possible undesired consequence is the harmless warning printed when
allproc linked list length does not match nprocs.

Diagnosed by: Svatopluk Kraus <onwahe@gmail.com>
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week