common: Change argument order of log_printhex.

* common/logging.c (log_printhex): Chnage order of args.  Make it
printf alike.  Change all callers.
* configure.ac: Add -Wno-format-zero-length

This makes it consistent with modern libgpgrt logging and thus eases
back porting from newer GnuPG versions which use libgpgrt logging.

• Signed-off-by: Werner Koch <wk@gnupg.org>