debuginfod: Check the DEBUGINFOD_URLS environment variable early in client.

debuginfod: Check the DEBUGINFOD_URLS environment variable early in client.

If the debuginfod-client isn't configured we should do as little
as possible. Simply return early with ENOSYS if no servers are
configured. This means we won't check

This does change the behavior of the debuginfod_find calls slightly.
Previously we would setup and check the cache if the given build-id
was valid. Which might have provided a result if an earlier client
had run with the same cache and valid server URLs which knew about
that particular build-id. Now we don't return any cached results
unless at least one server is configured.

This prevents selinux errors when the library is used in a confined
setup.

Signed-off-by: Mark Wielaard <mark@klomp.org>