svs: disabling via bootloader

[Maxime Villard <max%m00nbsd.net@localhost>]

I intend to change the way SVS can be disabled. Currently you can disable it by
typing: "sysctl -w machdep.svs.enabled=0". Once set to 0, it cannot be set back
to 1.

I want to make it a boot option in the bootloader. You will have to pass "-3"
in the "boot" command, like: "boot netbsd -3".

The reason I want to do that is because the current code to disable SVS is
really awful (hotpatching, etc...), and more importantly, there are
optimizations that could be possible if we knew SVS couldn't be turned off at
runtime. For example marking the trampoline pages as global (PTE_G), so that
they don't get flushed on each syscall. Or, on NVMM-Intel, we could make
MSR_LSTAR static and not save it each time.

The "machdep.svs.enabled" sysctl will remain available, but will be read-only,
and will just indicate whether SVS is enabled.