Save frequency to network config

Change-Id: Ib228271a7f0a51100e2088cba52bfc11a8ad3b61
wpa_supplicant: Add get_capability freq

Add "get_capability freq" command to print a more verbose list of channels and
    frequencies in MHz. The tag "NO_IBSS" is added, if IBSS mode is not allowed on
    a channel. This allows userspace programs to get the frequencies and
    limitations associated with each channel.

    This extends the information already provided in "get_capability channel" but a
    new interface is chosen because of backwards compatibility considerations.

Change-Id: Ia9ebeaad4f4d0cb64dc6cebc7ebb1eeca8a87ef3