airpcap-nl: implement AirpcapGetTxPower(), AirpcapSetTxPower

Use NL80211_CMD_SET_WIPHY with attributes NL80211_ATTR_WIPHY_TX_POWER_SETTING.
Keep track of maximum TX power for each channel in a new private attribute
in AirpcapChannelInfo when doing initial adapter channel query.
airpcap-nl: fix potential free of uninitialized pointers in handle allocation

Whoops! Thanks clang!
airpcap-nl: fix bug in AirpcapOpen in error case

If Ebuf is NULL and setting the initial channel fails,
we could copy the handle adapter error message into the NULL
pointer.

Thanks Clang static analyzer!
airpcap-nl: fix bug in AirpcapSetTxPower() with Power=0

This didn't do anything because we never got to the real
call to set the TX power in that branch.

Thanks again Clang static analyzer! :)