Further speed optimisations and backport some features

Some further speed optimisations by Russell Marks.

Further slight optimisations to bit-readers,

Backport some unzip 1.8 changes (buffer-overrun fixes, bit 7 strip on output filenames, less frequent ^C checking, and low-memory message), and add various comments.

Use self-modifying code with an unrolled end loop in rdbybits, and add a "rd1bit" macro, for about a 12% speed improvement on overall extraction time compared to version 1.5-3.

Fix long-filename buffer overrun, for filenames longer than 255 characters.

Backport Howard Goldstein's buffer-overrun fixes from unzip 1.8 - previously, a 255-byte buffer was repeatedly used to read up-to-65535-byte inputs.