kern: syscall_thread_enter() cannot fail

[kevans]

Summary

Attempting to handle the error gracefully can easily result in missing
SIGSYS, so this was made to always succeed in
39024a89146 ("syscalls: fix missing SIGSYS for several ENOSYS errors")
and returns the nosys entry on failure.

Drop the pretense of returning an error and clean up a few dead error
paths.

[kib]

sys/kern/kern_sig.c

2743

I prefer the place for the saved errno to be called nerror still. It is even useful that error would disappear.

[kevans]

error back to nerror