`scanadf -h` segfaults

Hi,

when I issue scanadf -h on Fedora (see bug) I got a segfault with backtrace.

It is caused by scanadf - it cleans up in bad way - it calls exit() instead having specific exit function as e.g. scanimage has. If I adjust the exit function from scanimage for scanadf, the segfault is gone.

I'll create a pull request soon.