Add a reattach command for fast reassociate-back-to-same-BSS

Add a reattach command for fast reassociate-back-to-same-BSS

Add "reattach" command to perform single-channel single-ssid scan
instead of full scan when trying to reconnect to the currently
"connected" network (assuming old scan results are not current enough to
skip the scan completely). This allows the scan result to come back in
much faster time. In ath9k, the scan took around 12 seconds with full
background scan, and only 0.1 second with the single-channel single-ssid
scan. Thus, take much less time for the client to re-establish
connection with the currently "connected" network.

Signed-hostap: Peter Qiu <zqiu@chromium.org>