Fix warning: 'va_start' has undefined behavior with reference types.

 - fixing the clang-flagged varargs issue
    * quick'n'dirty via boost preprocessor, long-term, we should
      e.g. use boost program_options
      (ceph_erasure_code already uses it anyway)
 - interesting bug found: ceph_authtool.cc:84 called
   ceph_argparse_flag with a std::string arg
 - only converted one of the 6 varargs functions, pending
   feedback

Signed-off-by: Thorsten Behrens <tbehrens@suse.com>