Fix detection of running of recent Claws Mail

Fix detection of running of recent Claws Mail

Starting in version 3.17.8-15-ge56346845 the socket path is built using
g_get_user_runtime_dir¹ insead of g_get_tmp_dir².

This change rendered the  current detection scheme invalid, though it's
still available as fallback to detect older Claws Mail versions.

This fix hopefully emulates that function by reading the same
environment variables.

¹ https://developer.gnome.org/glib/stable/glib-Miscellaneous-Utility-Functions.html#g-get-user-runtime-dir
² https://developer.gnome.org/glib/stable/glib-Miscellaneous-Utility-Functions.html#g-get-tmp-dir