hostapd: Add airtime policy configuration support

hostapd: Add airtime policy configuration support

This adds support to hostapd for configuring airtime policy settings for
stations as they connect to the access point. This is the userspace
component of the airtime policy enforcement system PoliFi described in
this paper: https://arxiv.org/abs/1902.03439

The Linux kernel part has been merged into mac80211 for the 5.1 dev
cycle.

The configuration mechanism has three modes: Static, dynamic and limit.
In static mode, weights can be set in the configuration file for
individual MAC addresses, which will be applied when the configured
stations connect.

In dynamic mode, weights are instead set per BSS, which will be scaled
by the number of active stations on that BSS, achieving the desired
aggregate weighing between the configured BSSes. Limit mode works like
dynamic mode, except that any BSS *not* marked as 'limited' is allowed
to exceed its configured share if a per-station fairness share would
assign more airtime to that BSS. See the paper for details on these
modes.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>