When trying to linking curve_keygen as if it was a C program, linking
fails (at least with gcc 4.8 on Debian testing) because it can't find
C++ symbols. Linking with g++ instead fixes the issue.

To achieve this, the source is renamed to curve_keygen.cpp, and
tools/Makefile.am is updated accordingly.
Signed-off-by: Gergely Nagy <algernon@balabit.hu>

The use of binary for CURVE keys is painful; you cannot easily copy
these in e.g. email, or use them directly in source code. There are
various encoding possibilities. Base16 and Base64 are not optimal.
Ascii85 is not safe for source (it generates quotes and escapes).

So, I've designed a new Base85 encoding, Z85, which is safe to use
in code and elsewhere, and I've modified libzmq to use this where
it also uses binary keys (in get/setsockopt).

Very simply, if you use a 32-byte value, it's Base256 (binary),
and if you use a 40-byte value, it's Base85 (Z85).

I've put the Z85 codec into z85_codec.hpp, it's not elegant C++
but it is minimal and it works. Feel free to rewrap as a real class
if this annoys you.