Add gdb/nat common functions for listing threads

Add gdb/nat common functions for listing threads

Add netbsd_nat::netbsd_thread_lister a generic thread lister, used
internally in netbsd-nat.c, copied from gdb/nbsd-nat.c.

Add public extern functions for listing threads:
 * netbsd_nat::thread_alive
 * netbsd_nat::thread_name
 * netbsd_nat::for_each_thread

gdb/ChangeLog:

	* netbsd-nat.h: Include "gdbsupport/function-view.h".
	* (netbsd_nat::thread_alive, netbsd_nat::thread_name)
	(netbsd_nat::for_each_thread): Add.
	* netbsd-nat.c: Include "gdbsupport/common-defs.h" and
	"gdbsupport/common-debug.h".
	* (netbsd_nat::netbsd_thread_lister)
	(netbsd_nat::thread_alive, netbsd_nat::thread_name)
	(netbsd_nat::for_each_thread): Add.