Fix some grammar and correct a manpage section

prodding and ok jmc@

Split up fork1():
- FORK_THREAD handling is a totally separate function, thread_fork(),
that is only used by sys___tfork() and which loses the flags, func,
arg, and newprocp parameters and gains tcb parameter to guarantee
the new thread's TCB is set before the creating thread returns
- fork1() loses its stack and tidptr parameters
Common bits factor out:
- struct proc allocation and initialization moves to thread_new()
- maxthread handling moves to fork_check_maxthread()
- setting the new thread running moves to fork_thread_start()
The MD cpu_fork() function swaps its unused stacksize parameter for
a tcb parameter.

luna88k testing by aoyama@, alpha testing by dlg@
ok mpi@