gdb: change frame_info::prev_func::p type to cached_copy_status

gdb: change frame_info::prev_func::p type to cached_copy_status

One might think that variable `frame_info::prev_func::p` is a simple
true/false value, but that's not the case, it can also have the value -1
to mean "unavaiable".  Change it to use the `cached_copy_status` enum,
which seems designed exactly for this purpose.

Rename to `status` to be consistent with `prev_pc::status` (and be cause
`p` means `predicate`, which implies boolean, which this is not).

gdb/ChangeLog:

	* frame.c (frame_info) <prev_func> <p>: Rename to status, change
	type to cached_copy_status.
	(fprintf_frame): Adjust.
	(get_frame_func_if_available): Adjust.
	(frame_cleanup_after_sniffer): Adjust.

Change-Id: I50c6ebef6c0acb076e25c741f7f417bfd101d953