checkoptions() that can be used to set defaults for this and other options
based on existing options.
Also involved some cleanups to how gen_wrapper is used.
holger reported that decode_utf8 was crashing with perl 5.8.8. Earlier, I
thought that passing 0 to the function avoided this with old perls, but
that was apparently not enough, it still crashes. So, put it inside the
eval, so we can at least recover from it crashing.
The old code actually did the same thing, just obfuscated -- since the eval
use wasn't quoted, it used the modules on load. Thus, the error (not to
mentioned the return) was bypassed, and it just failed on load.
But that seems like the right thing to do, really, so just made it clearer
that's what happens.