In cases where we're using $XDG_RUNTIME_DIR to create a predictable
control socket directory, stop setting the $GNOME_KEYRING_CONTROL
environment variable.

Note that we don't use the $XDG_RUNTIME_DIR fallback. This is because
two of our clients don't link in GLib, both the pam and pkcs11
modules. Getting involved in the whole tree of fallback possibilities
for how to resolve $XDG_RUNTIME_DIR is not something I'm interested
in duplicating.

So instead what we do is if $XDG_RUNTIME_DIR is not set, we fall back
to using the old $GNOME_KEYRING_CONTROL environment variable.

We use the GLib logic when looking for XDG_RUNTIME_DIR. The variable
is considered present even when empty.

https://bugzilla.gnome.org/show_bug.cgi?id=725801