PR27783: switch default debuginfod-urls to drop-in style files

PR27783: switch default debuginfod-urls to drop-in style files

Rewrote and commented the /etc/profile.d csh and sh script fragments
to take the default $DEBUGINFOD_URLS from the union of drop-in files:
/etc/debuginfod/*.urls.  Hand-tested with csh and bash, with
conditions including no prior $DEBUGINFOD_URLS, nonexistent .urls
files, multiple entries in .urls files.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>