Annotate syscall provider pointer arguments with the "userland" keyword.

Annotate syscall provider pointer arguments with the "userland" keyword.

This causes dtrace to automatically copyin arguments from userland, so
one no longer has to explicitly use the copyin() action to do so. Moreover,
copyin() on userland addresses is a no-op, so existing scripts should be
unaffected by this change.

Discussed with:	rstone
MFC after:	2 weeks