Add OpenBSD/sh native support.

Add OpenBSD/sh native support.
* NEWS (New native configurations): Mention OpenBSD/sh.
* configure.host: Add sh*-*-openbsd*.
* shnbsd-nat.c: Include "inf-ptrace.h".
(shnbsd_fetch_inferior_registers): Rename from
fetch_inferior_registers.  Make static.
(shnbsd_store_inferior_registers): Rename from
store_inferior_registers.  Make static.
(_initialize_shnbsd_nat): New function.
* Makefile.in (shnbsd-nat.o): Update dependencies.
* config/sh/nbsd.mh (NAT_CLIBS, NAT_FILE): Remove.
(NATDEPFILES): Remove infptrace.o and inftarg.o.  Add
inf-ptrace.o.