Add --enabled-debuginfod command line option

Add --enabled-debuginfod command line option

Currently debuginfod is enabled in Valgrind when the $DEBUGINFOD_URLS
environment variable is set and disabled when it isn't set.

This patch adds an --enable-debuginfod=<yes|no> command line option
to provide another level of control over whether Valgrind attempts
to download debuginfo. "yes" is the default value.

$DEBUGINFOD_URLS must still contain debuginfod server URLs in order
for this feature to work when --enable-debuginfod=yes.

https://bugs.kde.org/show_bug.cgi?id=453602