WNM: Use standard BSS selection and enable abridged bit handling

WNM: Use standard BSS selection and enable abridged bit handling

Most of the logic to reject BSSs during transition has been moved into
wnm_is_bss_excluded(). In addition to this, since commit 67bf89f55442
("WNM: Choose the best available BSS, not just the first one") we will
simply choose the BSS with the best throughput.

Overall, this matches the behavior that wpa_supplicant will use anyway
in wpa_supplicant_select_bss(). The only bigger difference is that using
this will check all known BSSs instead of only the ones in the candidate
list. This means that with this change the abridged bit is handled
according to standard.

There are some subtle changes to the logic. One is, that candidates with
a very low signal level are not explicitly dropped anymore. However,
that code pre-dates the logic to prefer the best BSS and should not be
relevant anymore.

Another small adjustment is to change the custom logic to avoid roaming
when it is not needed to use wpa_supplicant_need_to_roam_within_ess().

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>