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

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

o Fixes:
- When switching to 4-bit operation, send a SET_CLR_CARD_DETECT command
to disconnect the card-detect pull-up resistor from the DAT3 line before
sending the SET_BUS_WIDTH command.
- Add the missing "reserved" zero entry to the mantissa table used to
decode various CSD fields. This was causing SD cards to report that they
could run at 30 MHz instead of the maximum 25 MHz mandated in the spec.
o Enhancements:
- At the MMC layer, format various info from the CID into a string that
uniquely identifies the card instance (manufacturer number, serial
number, product name and revision, etc). Export it as an instance
variable.
- At the MMCSD layer, display the formatted card ID string, and also
report the clock speed of the hardware (not the card's max speed), and
the number of bits and number of blocks per transfer. It comes out like
this now:
mmcsd0: 968MB <SD SD01G 8.0 SN 276886905 MFG 08/2008 by 3 SD> at mmc0
22.5MHz/4bit/128-block
o Use DEVMETHOD_END.
o Use NULL instead of 0 for pointers.

PR: 156496
Submitted by: Ian Lepore
MFC after: 1 week

Loadable modules for mmc (the bus) amd mmcsd (the MMC and SD support
code). A little green, so not yet connected to the build.