Fix strdup_and_subst_obj() so that calls not involving simple labels collapse \\...

Fix strdup_and_subst_obj() so that calls not involving simple labels collapse \\ to \.
Without this, users cannot get a lone backslash in an HTML-label or tooltip if it
preceded G, E, N, etc.
We don't do this for simple label text, as it is later processed to handle \n, \r, etc.
and collapses \\ to \ at that time.