o separate configured beacon interval from listen interval; this
avoids potential use of one value for the other (e.g. setting
powersavesleep to 0 clobbers the beacon interval used in hostap
or ibss mode)
o bounds check the beacon interval received in probe response and
beacon frames and drop frames with bogus settings; not clear
if we should instead clamp the value as any alteration would
result in mismatched sta+ap configuration and probably be more
confusing (don't want to log to the console but perhaps ok with
rate limiting)
o while here up max beacon interval to reflect WiFi standard
Noticed by: Martin <nakal@nurfuerspam.de>
MFC after: 1 week
a71123cebf