Support for ffs "discard" option on SD/MMC cards

[Jared McNeill <jmcneill%invisible.ca@localhost>]

Hi folks --

I've added support for block erase to the ld_sdmmc driver in -current. 
This is activated by adding the "discard" mount option to your ffs 
partitions in /etc/fstab:

  /dev/ld0a       /               ffs     rw,discard      1 1

The mount man page says that the discard mount option is experimental, so 
YMMV. The option is there if you want to play with it on your SD cards and 
eMMC chips now.

Cheers,
Jared