Option to print window id; allow working with tools like urxvtc and single-instance chrome

Hello,

This is basically as the title says. I added an option to print the id of the discovered window for use with tools like [wmctrl](http://tomas.styblo.name/wmctrl/), and I removed the SIGCHLD handler. Both of these changes are towards making `xtoolwait` useful with single-instance applications like Firefox or `urxvtd` where all windows share the same pid, and where the command invoked by `xtoolwait` can (on a multicore CPU, anyway) map its window and exit before `fork()` even returns in the parent process.

Thanks for the handy tool!