PR25461: add /etc/profile.d files for default $DEBUGINFOD_URLS

PR25461: add /etc/profile.d files for default $DEBUGINFOD_URLS

To give distros a simple knob to let their userbase enjoy debuginfod
services, we add a --enable-debuginfod-urls[=URLS] to the configury.
This propagates to $DEBUGINFOD_URLS environment variable settings
in /etc/profile.d/debuginfod.{sh,csh} files.

The default is nothing.  If simply enabled, the current prototype
public server is used.  If enabled with parameters, the builder/distro
can specify a default URL search path of their own.

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