Normalize names of the NetBSD files

The files used to be named 'nbsd', which incorrectly reflects
the name of the OS and confuses it with other BSD derived OSes.

gdb/ChangeLog:

        * Makefile.in (ALL_64_TARGET_OBS, ALL_TARGET_OBS)
        HFILES_NO_SRCDIR, ALLDEPFILES): Rename files.
        * alpha-bsd-nat.c: Adjust include.
        * alpha-bsd-tdep.h: Adjust comment.
        * alpha-nbsd-tdep.c: Rename to ...
        * alpha-netbsd-tdep.c: ... this, adjust include.
        * amd64-nbsd-nat.c: Rename to ...
        * amd64-netbsd-nat.c: ... this, adjust include.
        * amd64-nbsd-tdep.c: Rename to ...
        * amd64-netbsd-tdep.c: ... this, adjust include.
        * amd64-tdep.h: Adjust include.
        * arm-nbsd-nat.c: Rename to ...
        * arm-netbsd-nat.c: ... this, adjust include.
        * arm-nbsd-tdep.c: Rename to ...
        * arm-netbsd-tdep.c: ... this, adjust include.
        * arm-nbsd-tdep.h: Rename to ...
        * arm-netbsd-tdep.h: ... this, adjust include.
        * configure.nat: Adjust file lists.
        * configure.tgt: Likewise.
        * hppa-nbsd-nat.c: Rename to ...
        * hppa-netbsd-nat.c: ... this, adjust include.
        * hppa-nbsd-tdep.c: Rename to ...
        * hppa-netbsd-tdep.c: ... this, adjust include.
        * i386-nbsd-nat.c: Rename to ...
        * i386-netbsd-nat.c: ... this, adjust include.
        * i386-nbsd-tdep.c: Rename to ...
        * i386-netbsd-tdep.c: ... this, adjust include.
        * m68k-bsd-nat.c: Adjust include.
        * mips-nbsd-nat.c: Rename to ...
        * mips-netbsd-nat.c: ... this, adjust include.
        * mips-nbsd-tdep.c: Rename to ...
        * mips-netbsd-tdep.c: ... this, adjust include.
        * mips-nbsd-tdep.h: Rename to ...
        * mips-netbsd-tdep.h: ... this.
        * nbsd-nat.c: Rename to ...
        * netbsd-nat.c: ... this, adjust include.
        * nbsd-nat.h: Rename to ...
        * netbsd-nat.h: ... this, adjust include.
        * nbsd-tdep.c: Rename to ...
        * netbsd-tdep.c: ... this, adjust include.
        * nbsd-tdep.h: Rename to ...
        * netbsd-tdep.h: ... this.
        * ppc-nbsd-nat.c: Rename to ...
        * ppc-netbsd-nat.c: ... this, adjust include.
        * ppc-nbsd-tdep.c: Rename to ...
        * ppc-netbsd-tdep.c: ... this, adjust include and comment.
        * ppc-nbsd-tdep.h: Rename to ...
        * ppc-netbsd-tdep.h: ... this.
        * sh-nbsd-nat.c: Rename to ...
        * sh-netbsd-nat.c: ... this, adjust include.
        * sh-nbsd-tdep.c: Rename to ...
        * sh-netbsd-tdep.c: ... this, adjust include.
        * sparc-nbsd-nat.c: Rename to ...
        * sparc-netbsd-nat.c: ... this.
        * sparc-nbsd-tdep.c: Rename to ...
        * sparc-netbsd-tdep.c: ... this, adjust include.
        * sparc64-nbsd-nat.c: Rename to ...
        * sparc64-netbsd-nat.c: ... this.
        * sparc64-nbsd-tdep.c: Rename to ...
        * sparc64-netbsd-tdep.c: ... this, adjust include.
        * sparc64-tdep.h: Adjust comment.
        * vax-bsd-nat.c: Adjust include.
        * vax-nbsd-tdep.c: Rename to ...
        * vax-netbsd-tdep.c: ... this, adjust include.