query: Change internal result computation

While looking at the query library, I realized that while we have five
field statuses, making this a 5-dimensional space, four of them are
shrunk to a single possible value (None). Hence it should be possible to
convert this into a single value space plus extra 4 special constants.

This patch implements this, making (IMHO) the return value of normal
functions much simpler: you simply return the desired value, instead of
(QRFS_NORMAL, value); for the special results, you simply return
_FS_UNAVAIL, instead of (QRFS_UNAVAIL, None). This I believe does
simplify the code.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>