wlanwatch: clean up to compile / run

• add static where needed
• delete old unused flags, dating back to the initial commit from Sam
• indirect through void * where appropriate, to avoid LLVM alignment errors
• PRIu64 where needed

This does work, but it currently doesn't track the ifname itself,
so we see ifindexes, not ifnames:

Wed Feb 26 18:35:59 RTM_IEEE80211: if# 2, scan complete
Wed Feb 26 18:35:59 RTM_IEEE80211: if# 2, associate with 30:de:4b:db:46:0a
Wed Feb 26 18:35:59 RTM_IFINFO: if# 2, link: up, flags:<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST>

.. but it does work!

Differential Revision: https://reviews.freebsd.org/D49149