Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r253757:
Fix alignment of USB WLAN radiotap headers. This makes USB WLAN adapters
work on ARM, MIPS and similar platforms, where alignment matters.

MFC r246614:
- Streamline detach logic in wlan drivers, so that
freed memory cannot be used during detach.
- Remove all panic() calls from the urtw driver because
panic() is not appropriate here.
- Remove redundant checks for device detached in
device detach callbacks.
- Use DEVMETHOD_END to mark end of device methods.

MFC r244503 and r246565:

Make sure all USB drivers allocate buffer memory
through the USB API and/or busdma.

The following assumptions have been made:
umass - buffers passed from CAM/SCSI layer are OK
network - mbufs are OK.

Some other nits while at it.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

adds sysctl knobs to show rate statistics that it could be useful to
debug slow TX speed.

overhauls urtw(4) for supporting RTL8187B devices properly that there
was major changes to initialize RF chipset and set H/W registers and
removed a lot of magic numbers on code. Details are as follows:

- uses the endpoint 0x89 to get TX status information which used to
get TX complete or retry numbers or get a beacon interrupt. It's
only valuable for RTL8187B.
- removes urtw_write[8|16|32]_i functions that it's useless now.
- uses ic->ic_updateslot to set SLOT, SIFS, DIES, EIFS, CW_VAL
registers that doesn't set these whenever the channel is changed.
- code for initializing RF chipset for RTL8187B changed a lot that
there was many problems on TX transfers so it doesn't work properly
even if just for a ping/pong. Now it becomes more stable than
before that TX throughputs using netperf(1) were about 15 ~ 17Mbps/s
though sometimes it encounters packet losses.
- removes a lot of magic numbers that in the previous all of
representing RX and TX descriptors were consisted of magic numbers
and structures. It'd be more readable rather than before.
- calculates TX duration more accurately for urtw(4) devices.
- style(9)

s/usb2_/usb_/ on all typedefs for the USB stack.

s/usb2_/usb_/ on all C structs for the USB stack.

ports urtw(4) for USB2. Additionally it supports a 8187B chipset weakly
that it needs more stabilization.