Slight tweak to fork1() calling conventions. Add a third argument so

Slight tweak to fork1() calling conventions.  Add a third argument so
the caller can easily find the child proc struct.  fork(), rfork() etc
syscalls set p->p_retval[] themselves.  Simplify the SYSINIT_KT() code
and other kernel thread creators to not need to use pfind() to find the
child based on the pid.  While here, partly tidy up some of the fork1()
code for RF_SIGSHARE etc.