tools/net80211: add mlme_assoc

mlme_assoc is a tool to trigger net80211::ieee80211_sta_join1() calls
which in certain conditions cause problems to the LinuxKPI 802.11 compat
code (but also believed to possibly cause problems in case of race to
other firmware based drivers). This has proven to be a good reproducer
for the problem even on setups which otherwise could run for days without
hitting it.

Sponsored by: The FreeBSD Foundation
PR: 271979

(cherry picked from commit 643d6dce6c1e39f067f8d0feea8615913b324891)