Use startswith, instead of ==, when testing sys.platform to support
new platforms like Linux 3 or OpenBSD 5.

#12266: Fix str.capitalize() to correctly uppercase/lowercase titlecased and cased non-letter characters.

between 2**31 and 2**63 on systems with a 64-bit C "long".

Reported by Oleg Oshmyan in issue #12646.

This will make it easier to detect truncated input streams.

Also, make zlib's error messages more consistent.