debuginfod/debuginfod.cxx:
- Added default value to fdcache_prefetch_mds and
  fdcache_prefetch_fds when -R is specified. Defaults   to one half of
  fdcache's values for those respective   variables. These values are only set
  when -R is specified

Note: the ratio of prefetch:fdcache file descriptors and MBs
          is set to 1:2 since the fdcache seems to be used less
          than the standard fdcache.