gnu-nat: Move local functions inside gnu_nat_target class

This allows to have the process_stratum_target object at hand for future use in
the gdb API, and only use gnu_target from external calls.

gdb/Changelog:

	* gnu-nat.h (inf_validate_procs, inf_suspend, inf_set_traced,
	steal_exc_port, proc_get_state, inf_clear_wait, inf_cleanup,
	inf_startup, inf_update_suspends, inf_set_pid, inf_steal_exc_ports,
	inf_validate_procinfo, inf_validate_task_sc, inf_restore_exc_ports,
	inf_set_threads_resume_sc, inf_set_threads_resume_sc_for_signal_thread,
	inf_resume, inf_set_step_thread, inf_detach, inf_attach, inf_signal,
	inf_continue, make_proc, proc_abort, _proc_free, proc_update_sc,
	proc_get_exception_port, proc_set_exception_port, _proc_get_exc_port,
	proc_steal_exc_port, proc_restore_exc_port, proc_trace): Move functions
	to gnu_nat_target class.
	* gnu-nat.c: Likewise.
	(inf_update_procs, S_proc_wait_reply, set_task_pause_cmd,
	set_task_exc_port_cmd, set_signals_cmd, set_thread_pause_cmd,
	set_thread_exc_port_cmd): Call inf_validate_procs through gnu_target
	object.
	(gnu_nat_target::create_inferior, gnu_nat_target::detach): Pass `this'
	instead of `gnu_target'.