Fixed interrupt unmasking for child processes which I broke in

rev.1.10 two years ago.  Children continued to run at splhigh()
after returning from vm_fork().  This mainly affected kernel
processes and init.  For ordinary processes, interrupts are normally
unmasked a few instructions later after fork() returns (it may be
important for syscall() not to reschedule the child processes).
Kernel processes had workarounds for the problem.  Init manages to
start because some routines "know" that it is safe to go to sleep
despite their caller starting them at a high ipl.  Then its ipl
gets fixed on its first normal return from a syscall.

sys/kern/kern_fork.c

@@ -343,6 +343,7 @@ fork1(p1, flags, retval)
 		 * Child process.  Set start time and get to work.
 		 */
 		microtime(&runtime);
+		(void) spl0();
 		p2->p_stats->p_start = runtime;
 		p2->p_acflag = AFORK;
 		return (0);