makesyscalls: don't make syscall.mk by default

We only want to produce syscall.mk for the main syscall table so default
to not producing it (send it to /dev/null) and add a syscalls.conf to
sys/kern to trigger the creation of sys/sys/syscall.mk. This eliminates
the need for entries in other syscalls.conf files and is a cleaner
pattern going forward.

Reviewed by: kevans, imp
Differential Revision: https://reviews.freebsd.org/D42663