Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Cleanups to prepare this code for wider use (likely merged into hostapd):
o add (required) cmd line args to specify the set of ifnet's to monitor
for WDS discovery msgs; "any" is a wildcard
o change the default script run on wds vap create to the "null script"
o auto-daemonize; add -f option to force foreground operation
o add -P option for integration with rc.d (implementation missing, tba)
o use syslog; default to log up to LOG_INFO, -t (terse) gives you up to
LOG_ERR, and -v (verbose) gives you up to LOG_DEBUG
o scan for existing vaps on startup to recover existing state
o correct some types

o remove the -p option; use the %parent pointer sysctl to find the parent ifnet
o remove the -b option; users can just embed the bridge ifnet in the script
o add -j option to create wds vaps on join/rejoin ops

add dwds test app

Supported by: Hobnob
Obtained from: Atheros (earlier version)