[syscalls] Add ZX_POL_NEW_PROCESS

Previously, we lacked a policy bit for controlling process creation.
We'll use this policy bit in the future to force clients through
fuchsia.process.Launcher.

Also, hide ZX_POL_MAX from userspace so we can add more policy bits in
the future.

TEST=spawn_launcher_test

Change-Id: Idcba7c7db595774bd10b227da41e4c7249bb83ea