bson_api.3 and bson_index.3 man pages include all other pages, so
skip them.

Thanks to Petr Pisar for the patch.

Previously we had a thread per test; once the number exceeded circa 250,
test-libbson --threads failed on MIPS.

Also standardize parsing of "NaN" and fix CDRIVER-1978: standard output
of NaN and Infinity.