Add symlink drive mappings as mount options.

This introduces control over symlink prefixes so that the user can map a
Windows drive letter to a mounted path in the Unix filesystem hierarchy,
preserving cross-filesystem symlinks.

The mappings are specified in the mount option format (example):
    drive_d=/mnt/data_drive
...which maps drive D: in absolute symlink targets to the Unix
filesystem path '/mnt/data_drive'.