Make "catch fork" work on systems with PT_GET_PROCESS_STATE

Make "catch fork" work on systems with PT_GET_PROCESS_STATE

These systems (OpenBSD and HP-UX 10.x) already support follow-fork
including the events needed to for "catch fork".  This just makes
the upper layers realize this.

gdb/
2015-03-04  Mark Kettenis  <kettenis@gnu.org>

        * inf-ptrace.c [PT_GET_PROCESS_STATE]
        (inf_ptrace_insert_fork_catchpoint): New function.
        (inf_ptrace_remove_fork_catchpoint): New function.
        (inf_ptrace_target) [PT_GET_PROCESS_STATE]: Install them.