Restructured tags to better support xored-globals

32-bit tag structure:
[---        32       ---]
[1|- 9 -|- 10 -|-- 12 --]
 ^   ^     ^       ^- entry length
 |   |     \--------- file id
 |   \--------------- tag type
 \------------------- valid

In this tag, the type decomposes into some more information:
[---      9      ---]
[1|- 2 -|- 3 -|- 3 -]
 ^   ^     ^     ^- struct
 |   |     \------- type
 |   \------------- scope
 \----------------- user

The change in this encoding is the addition of a global scope:
LFS_SCOPE_STRUCT = 0 00 xxx xxx
LFS_SCOPE_ENTRY  = 0 01 xxx xxx
LFS_SCOPE_DIR    = 0 10 xxx xxx
LFS_SCOPE_FS     = 0 11 xxx xxx
LFS_SCOPE_USER   = 1 xx xxx xxx

Author: geky