file-ids: Enlighten ID generation and storage

The unique id is 32 bit and consists only of the subid value. This
is _really_ enough. The genid part is just a hint for the tree-search
algirythm to avoid unneeded sys_kcmp calls.

Plus, generate IDs for special files. This will make it easier to
move the regfiles into into separate files (see the respective patch
for details).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>