More crash catcher fixes

[Chris]

Here's another patch for the crash catcher. It aims to fix a number of issues and potential problems. It removes a bunch of potentially-unsafe function calls from the signal handler, and sets it up so the forked process invokes another run of ZDoom with a special switch, which makes it read a bunch of info from the crashed process to do the majority of the work safely, then exit. It also fixes a problem with some distros that prevented GDB from attaching to the crashed process.