6.3.2: Fixes but also some real changes

It's mostly small fixes, but some of these changes may affect your application. So test carefully.

• Conversion errors no longer throw pqxx::failure; always conversion_error!
• Use C++17's built-in numeric string conversions, if available.
• Query numeric precision in a more sensible, standard way.
• Avoid "dead code" warning.
• Replace obsolete autoconf macros.
• Remove all using namespace std.
• CMake build fixes.