Move OpenBSD-only functions from inf-ptrace to obsd-nat

Move OpenBSD-only functions from inf-ptrace to obsd-nat

All major BSDs implement PT_GET_PROCESS_STATE, but they differ in
details and want to implement follow-fork functionality differently.

gdb/ChangeLog:

	* inf-ptrace.h (follow_fork, insert_fork_catchpoint)
	(remove_fork_catchpoint, post_startup_inferior)
	(post_attach): Move...
	* obsd-nat.h (follow_fork, insert_fork_catchpoint)
	(remove_fork_catchpoint, post_startup_inferior)
	(post_attach): ...here.
	* inf-ptrace.c (follow_fork, insert_fork_catchpoint)
	(remove_fork_catchpoint, post_startup_inferior)
	(post_attach): Move...
	* obsd-nat.c (follow_fork, insert_fork_catchpoint)
	(remove_fork_catchpoint, post_startup_inferior)
	(post_attach): ...here.