Fix header comment for bitncmp().

The result is an int less than, equal to, or greater than zero, in the
style of memcmp (and, in fact, exactly the output of memcmp in some cases).
This comment previously said -1, 1, or 0, which was an overspecification,
as noted by Emre Hasegeli.  All of the existing callers appear to be fine
with the actual behavior, so just fix the comment.

In passing, improve infelicitous formatting of some call sites.