Support rthreads on OpenBSD 5.2 and later.

Support rthreads on OpenBSD 5.2 and later.

OpenBSD 5.2 and later have a proper threads implementation based on
kernel threads.  Debugging support is provided through additional
ptrace(2) requests, so this diff extends the generic code in
inf-ptrace.c with OpenBSD-specific code to discover additional threads.

gdb/ChangeLog:

        * obsd-nat.h: New file.
        * obsd-nat.c: New file.
        * Makefile.in (HFILES_NO_SRCDIR): Add obsd-nat.h.
        (ALLDEPFILES): Add obsd-nat.c.