NetBSD/m68k support, based on work by mikeb@snow.datametrics.com:

NetBSD/m68k support, based on work by mikeb@snow.datametrics.com:
* config.bfd (m68*-*-netbsd*): Use m68k-nbsd as bfd_name.
* configure.in (m68knetbsd_vec): Added.
* targets.c (bfd_m68knetbsd_vec): Added.
* hosts/m68knbsd.h, config/m68k-nbsd.mt, m68knetbsd.c: New files.
* Makefile.in (BFD32_BACKENDS, CFILES): Add m68knetbsd.c.

miscellaneous cleanup required by all netbsd targets, based on work
by Andrew Cagney <cagney@highland.com.au>:
* netbsd.h (N_MAGIC, N_SET_MAGIC, N_GETMAGIC, N_GETMAGIC2, N_TXTADDR,
  N_TXTOFF, N_ALIGN, N_DATADDR, N_DATOFF): Removed.  Generic a.out
  definitions work.
* i386nbsd.c, ns32knbsd.c, sparcnbsd.c (__LDPGSZ): Removed.
  (MY(write_object_contents)): Use NetBSD's magic numbers