libdebuginfod: Add the default progress callback function to the public API

libdebuginfod: Add the default progress callback function to the public API

Rename default_progressfn to debuginfod_default_progressfn and expose it
in libdebuginfod's public API.  Previously this function could only be
indirectly invoked by clients by setting the DEBUGINFOD_PROGRESS
environment variable and not explicitly setting a progress callback
function with debuginfod_set_progressfn.

The purpose of this change is to make debuginfod_default_progressfn
directly callable so that custom progress callback functions can
delegate progress reporting to it.

Signed-off-by: Aaron Merey <amerey@redhat.com>