[Libpqxx-general] Support char & unsigned char types

[Maxime van Noppen]

Maxime van Noppen wrote:
> those two types. As it is quite straightforward I did the patch and it
> works well for me (see attached file).

I found at least a bug with this patch (not so straightforward it seems
:)). I was (too) lazy and just casted the (unsigned) char into an
(unsigned) int to reuse the to_string_(un)signed functions but this
doesn't produce the expected output.

-- 
Maxime