sbr/vector.c: Change bvector_bits() to return first word.

sbr/vector.c: Change bvector_bits() to return first word.

Rename it to bvector_first_bits() to represent its new behaviour.  It
has only one caller that uses it to produce debug.  Tighten the API so
other callers don't get access to the location of the bvector's bits.