hostapd: introduce min_tx_power option

hostapd: introduce min_tx_power option

Introduce a new option min_tx_power to configure the minimum permitted
max TX power (in dBm) for ACS and DFS channel selection.

Imagine the following regulatory rules:
  * 5180 MHz [36] (21.0 dBm)
  * 5200 MHz [40] (21.0 dBm)
  * 5220 MHz [44] (21.0 dBm)
  * 5240 MHz [48] (21.0 dBm)
  * 5260 MHz [52] (20.0 dBm) (radar detection)
  * 5280 MHz [56] (20.0 dBm) (radar detection)
  * 5300 MHz [60] (20.0 dBm) (radar detection)
  * 5320 MHz [64] (20.0 dBm) (radar detection)
  * 5500 MHz [100] (21.0 dBm) (radar detection)
  * 5520 MHz [104] (21.0 dBm) (radar detection)
  * 5540 MHz [108] (21.0 dBm) (radar detection)
  * 5560 MHz [112] (21.0 dBm) (radar detection)
  * 5580 MHz [116] (21.0 dBm) (radar detection)
  * 5600 MHz [120] (21.0 dBm) (radar detection)
  * 5620 MHz [124] (21.0 dBm) (radar detection)
  * 5640 MHz [128] (21.0 dBm) (radar detection)
  * 5660 MHz [132] (21.0 dBm) (radar detection)
  * 5680 MHz [136] (21.0 dBm) (radar detection)
  * 5700 MHz [140] (21.0 dBm) (radar detection)
  * 5720 MHz [144] (13.0 dBm) (radar detection)
  * 5745 MHz [149] (13.0 dBm)
  * 5765 MHz [153] (13.0 dBm)
  * 5785 MHz [157] (13.0 dBm)
  * 5805 MHz [161] (13.0 dBm)
  * 5825 MHz [165] (13.0 dBm)
  * 5845 MHz [169] (13.0 dBm)
  * 5865 MHz [173] (13.0 dBm)

When ACS or DFS end up selecting channel 144 or higher, some clients
might no longer be able to communicate with the AP due to the TX power
being limited to 13 dBm. Setting min_tx_power to 20 will result in
hostapd not considering these channels during ACS or after a DFS event.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: David Bauer <mail@david-bauer.net>