Fixing signed/unsigned comparison warnings

Clang turned up some signed/unsigned comparison warnings. These warnings
have been fixed by cleaning up sdcard slightly:
- Don't use negative numbers for invalid gid/uid.
- sdcard takes a fixed number of arguments now so assert on that instead
  of using a for loop.
  - Also fixed usage string to reflect this fact.

Change-Id: Iee58a8e9aaedb3d40ad7dfeef63d8cd1fe1cd248
Author: Edwin Vane <edwin.vane@intel.com>
Reviewed-by: Kevin P Schoedel <kevin.p.schoedel@intel.com>