Modernize DEBUG macros, and make code -Wall -Werror clean

This involved several things:
- Remove unused variables
- Mark variables that may or may not be used when specific macros are
  defined, but which we still want to define for the other cases without
  having to #ifdef too much, as such by way of G_GNUC_UNUSED.
- use varargs macros instead of the DEBUG[2-5] ones (which we can now do
  since we require a C99 compiler).
- cast integers to what we request in the format string, and
  standardize on assuming that size_t is an unsigned int everywhere.
  This may not always be correct (depending on whether we do LFS
  support), but debugging statements that are not 100% correct (as long
  as they contain a value that maps one-on-one to the right bit pattern)
  isn't a huge problem.
- Move the "include cliserv.h" to below the "include glib.h", so we
  don't define macros twice.