[Libreoffice] SAL_WARN_UNUSED_RESULT

I've added a "SAL_WARN_UNUSED_RESULT" to sal/types.h which can be used
to decorate functions/methods whose return value *really really* must be
checked. It causes gcc to output a warning if the return type is
ignored. The cool kids build with --enable-werror so it's a build-time
error for them.

Using sparingly. Typically for methods equivalent to read where basing
decisions off data thought to be read, but not, would be a very bad
thing.

C.