(ns32k-*-netbsd): use ns32knbsd.
* hosts/{nbsd,i386nbsd,ns32knbsd,sparcnbsd}.h: New files, NetBSD
host support.
* sparcnetbsd.c: New file, adds support for NetBSD/sparc.
* config/sparc-nbsd.mt: Likewise.
* netbsd.h: New file, definitions common to all netbsd ports.
* i386netbsd.c: Use it.
* ns32knetbsd.c: Use it.
* Makefile.in, configure.in, targets.c, config/i386-nbsd.mt,
config/ns32k-nbsd.mt: canonicalize netbsd targets to conform to
file/variable naming conventions.
* i386netbsd.c: New file, renamed from netbsd386.c.
* ns32knetbsd.c: New file, renamed from netbsd532.c.
