Proper way to add vendor-specific syscalls?

[Alan Somers]

What's the proper way to add a vendor-specific syscall? The comments
in kern/syscalls.master suggest that they should be put in the range
from 151-180, but most of that range is actually occupied.  Only five
nosys slots are available.  If I add syscalls to the end of the list,
they'll likely collide with future standard syscalls.  Should I just
added ~100 nosys syscalls to the end of the list, and put my custom
syscalls afterwards?  Is there any penalty to lengthening the list?

-Alan