Lessons from trying to checkpoint Apache

This includes patches that add warnings/comments about some of the things that users need to be wary of when testing their applications with DMTCP. We faced these problems when testing DMTCP with Apache-2.2.
I can change this to a comment for now.

As I mentioned in the commit message, a future patch can include a method to do this automatically by wrapping the call to send SIGWINCH in an ifdef/if case that gets
disabled with a configure option/run-time option  (--no-sigwinch). Setting a flag based on the application type is another alternative but it's a little less elegant in my opinion -- we don't want to modify the core for one specific application/set of applications, and instead delegate the responsibility to the user.