vt(4): allow up to _SIG_MAXSIG (128) for VT_SETMODE

[quentin.thebault_defenso.fr]

Summary

VT_SETMODE ioctl currently checks the provided signal numbers with its own
ISSIGVALID macro that uses NSIG (32) as a maximum, although the code that will
actually send the signal in sys/kern/kern_sig.c uses _SIG_VALID which allows up
to _SIG_MAXSIG (128).

This change aligns the vt code with the kernel internals and enables the use of
higher signal numbers so that applications are not limited to SIGUSR1 and
SIGUSR2 for vt release and acquire signals.

Signed-off-by: Quentin Thébault <[e-mail]>

[kevans]

I know ray@ pops in sometimes so we might give him (and Ed) a little time to object, but I also didn't really see a reason to impose this particular limit. I did wonder if it was just an oversight because NSIG on other platforms *does* usually cover all valid signals (as far as I've observed)

[imp]

This looks good to my eye as well. We're just using kern_psignal(), which doesn't care. If there's some reason to avoid them, it's going to be subtle enough we should document the why.