Avoid integer overflow in batv-milter

The overflows were neither likely nor dangerous, but it's best to be on
the safe side.
Substantially update the documentation
Makefile: allow CXXFLAGS/LDFLAGS to be set in environment

This will allow the Debian packaging to specify build flags from
dpkg-buildflags.
Fix documentation of X-Batv-Status header

It uses a comma, not a semi-colon.
Release version 0.5