Measure Bloom index signature-length reloption in bits, not words.

Per discussion, this is a more understandable and future-proof way of
exposing the setting to users.  On-disk, we can still store it in words,
so as to not break on-disk compatibility with beta1.

Along the way, clean up the code associated with Bloom reloptions.
Provide explicit macros for default and maximum lengths rather than
having magic numbers buried in multiple places in the code.  Drop
the adjustBloomOptions() code altogether: it was useless in view of
the fact that reloptions.c already performed default-substitution and
range checking for the options.  Rename a couple of macros and types
for more clarity.

Discussion: <23767.1464926580@sss.pgh.pa.us>