Use a real vfork done event on FreeBSD when available.

Use a real vfork done event on FreeBSD when available.

FreeBSD 12 recently added a new ptrace event to indicate when the vfork
parent resumes after the child process stops sharing the address space.
Use this event to report a proper TARGET_WAITKIND_VFORK_DONE rather than
faking a vfork done event after a delay.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_enable_proc_events): Enable "PTRACE_VFORK"
	events.
	(fbsd_pending_vfork_done): Only define if "PTRACE_VFORK" is not
	defined.
	(fbsd_add_vfork_done): Likewise.
	(fbsd_is_vfork_done_pending): Likewise.
	(fbsd_next_vfork_done): Likewise.
	(fbsd_resume): Only ignore pending vfork done events if
	"PTRACE_VFORK" is not defined.
	(fbsd_wait): Only look for pending vfork done events if
	"PTRACE_VFORK" is not defined.
	[PTRACE_VFORK]: Handle "PL_FLAG_VFORKED" and "PL_FLAG_VFORK_DONE"
	events.
	(fbsd_follow_fork): Only fake a vfork done event if "PTRACE_VFORK"
	is not defined.