sdcard: a program to create a "virtual" /sdcard pointed at a path

sdcard is a program that uses FUSE to emulate FAT-on-sdcard style
directory permissions (all files are given fixed owner, group, and
permissions at creation, owner, group, and permissions are not
changeable, symlinks and hardlinks are not createable, etc.

usage: sdcard <path> <uid> <gid>

It must be run as root, but will change to uid/gid as soon as it
mounts a filesystem on /sdcard.  It will refuse to run if uid or
gid are zero.

Change-Id: I9a5d2e5daaebeee632f8470172cbb77b7fa689f8
Signed-off-by: Brian Swetland <swetland@google.com>