- pass return values in set{,Bool,Int,Double}Value() and setprop()

- pass return values in set{,Bool,Int,Double}Value() and setprop()
- make some char* "const" to silence warnings

This removes the warning that (rarely) occurred if one wrote to a
write protected property with setprop(). While this was a useful
hint, it needlessly floods the terminal if one protected a property
intentionally. (Consider to add an SG_DEBUG warning instead.) It's
now the caller's job to check for the result if it actually cares.