com_check(): Add attribute checking.

The afs info stored in the audio file table contains the attribute
bit mask of each audio file. If there is a bit set which does
not correspond to an attribute defined in the attribute table, we
have an inconsistency. This commit adds a check that reports such
inconsistencies.

com_check(), which is part of afs.c, calls the attribute check
callback of attribute.c via the callback mechanism. The callback
computes the logical or of all defined bits and passes this bit mask
to aft_check_attributes() of aft.c to check each audio file against
the mask. Hence two new public functions are required.