[PATCH 2/5] mountmgr.sys: Simplify query_unix_drive.

[Erich E. Hoover]

This patch replaces query_dos_device() with a find_volume_by_letter()
routine and query_unix_device() with a find_volume_by_unixdev()
routine, which sets up for patch 3 to be able to share this code to
find the volumes (by letter and unix device).  The patch also
consolidates all the actual query code into query_unix_drive.

Best,
Erich