TS-3070: consistent span configuration across platforms

  - Remove unused Span::isRaw.
  - Add ink_file_is_mmappable().
  - Add ink_file_get_geometry().
  - Make Span geometry variables unsigned.
  - Use the symbolic constant for the Linux raw device major number.
  - Allow file spans to omit the size configuration parameter.
  - Use a scoped file descriptor to read Span configuration.
  - Consolidate span configuration so that it is the same for all
    platforms. Use ink_file_get_geometry to abstrct the method of
    probing block device geometry.
  - Use the inode number for the disk_id of cache files so that
    multiple cache files can be used.
  - Remove global Store theStore. The global Store theStore is not
    used for anything that matters. The cache and hostdb subsystems
    have their own separate Store instances.