lib: Make sid_parse return the parsed length

lib: Make sid_parse return the parsed length

Use a temporary struct as a return value to make the compiler catch all
callers. If we just changed bool->ssize_t, this would just generate a
warning. struct sid_parse_ret will go away in the next commit

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>