Reserve gaps in syscall numbers for local use

It is best for auditing of syscalls.master if we only append to the
file. Reserving unimplemented system call numbers for local use makes
this policy and provides a large set of syscall numbers FreeBSD
derivatives can use without risk of conflict.

Reviewed by: jhb, kevans, kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27988