Add support for specifying subset of enabled frequencies to scan

Add support for specifying subset of enabled frequencies to scan

A new network block parameter, scan_freq, can be used to specify subset
of frequencies to scan. This can speed up scanning process considerably
if it is known that only a small subset of channels is actually used in
the network. A union of configured frequencies for all enabled network
blocks is used in scan requests.

Currently, only driver_nl80211.c has support for this functionality.

For example, following parameter marks 2.4 GHz channels 1, 6, 11 to be
scanned: scan_freq=2412 2437 2462