Consolidate code to enable optional FreeBSD native target event reporting.

Consolidate code to enable optional FreeBSD native target event reporting.

Add a new function to enable optional event reporting for FreeBSD native
targets.  Specifically, use this to enable fork and LWP events.
The bodies of fbsd_enable_follow_fork and fbsd_enable_lwp_events have been
subsumed into the new function.  In addition, use the PT_GET_EVENT_MASK
and PT_EVENT_SET_MASK requests added in FreeBSD 12 when present to enable
these events.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_enable_lwp_events): Remove function.
	(fbsd_enable_proc_events): New function.
	(fbsd_enable_follow_fork): Remove function.
	(fbsd_post_startup_inferior): Use "fbsd_enable_proc_events".
	(fbsd_post_attach): Likewise.