MFC r344990:
Fix ieee80211_radiotap(9) usage in wireless drivers:

- Alignment issues:
* Add missing __packed attributes + padding across all drivers; in
most places there was an assumption that padding will be always
minimally suitable; in few places - e.g., in urtw(4) / rtwn(4) -
padding was just missing.
* Add __aligned(8) attribute for all Rx radiotap headers since they can
contain 64-bit TSF timestamp; it cannot appear in Tx radiotap headers, so
just drop the attribute here. Refresh ieee80211_radiotap(9) man page
accordingly.

- Since net80211 automatically updates channel frequency / flags in
ieee80211_radiotap_chan_change() drop duplicate setup for these fields
in drivers.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

rtwn, urtwn: drop unused structures.

urtwn(4) uses another implementation of command queue; rtwn(4) don't need
it at all.

urtwn, rtwn, rsu: switch to ieee80211_add_channel_list_2ghz().

- Use device's channel list instead of default one (from
ieee80211_init_channels()); adds 12 - 14 2GHz channels.
- Add ic_getradiocaps() method.

rtwn: replace hardcoded rate indices with their names (similar to r289758).

- Use macros to define rate indices; don't hardcode them in code.
- Add method for 'rate' -> 'rate index' conversion
(and array for the opposite).
- Determine if rate is CCK / OFDM via appropriate macro.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D4837

net80211: replace internal LE_READ_*/LE_WRITE_* macro with system
le*dec / le*enc functions.

Replace net80211 specific macros with system-wide bytestream
encoding/decoding functions:
- LE_READ_2 -> le16dec
- LE_READ_4 -> le32dec
- LE_WRITE_2 -> le16enc
- LE_WRITE_4 -> le32enc

+ drop ieee80211_input.h include, where it was included for these
operations only.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D6030

rtwn: use ieee80211_restart_all() for device reset

Tested by: kevlo
Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5057

[rtwn] bring over initial rtwn driver.

This is a port from openbsd. It's incomplete and unstable, but it's better
than nothing. I have no plans to MFC this until it's complete and stable.

Submitted by: kevlo