NetBSD doesn't ship with <machine/npx.h>.

NetBSD doesn't ship with <machine/npx.h>.

This is first step towards stopping to assume that HAVE_BSD_REGS_T implies
FreeBSD-style code. This breaks on NetBSD in several places.

Perhaps HAVE_BSD_REGS_T should be renamed to HAVE_FREEBSD_REGS_T.

Check headers with this fashion:
- FreeBSD requires <sys/types.h> for <machine/npx.h>
- FreeBSD requires <sys/cdefs.h> for <machine/reg.h>
- NetBSD doesn't need extra includes for <machine/reg.h>
- NetBSD requires <sys/param.h> before inclusion of <sys/cdefs.h>