json: Add file handling for debug output

* src/gpgme-json.c (main): Add possibilty to set log file.

This is similar to how GPGME_DEBUG works. It takes ; on
Windows as seperator and : on linux. Followed by a file name.

For Linux it might make sense to use a different seperator
to allow setting a socket explicitly. But this is better
in line with the current GPGME_DEBUG behavior.

The change helps on Windows where we don't have a
log socket.