Deprecate withProcess

[snoyberg]

I've now twice been bitten by a bug around withProcess:

• Use withProcess
• Stream data from child process
• Done

Problem: this creates a really subtle race condition where the child process may close its output handles and, before actually exiting, the parent process will hit closeProcess and then send a SIGTERM to the child.

The real problem is that withProcess is confusing in how it behaves. I propose instead having two new functions:

• withProcessTerm, with the current behavior
• withProcessWait, which will never send a SIGTERM, but instead wait for the child process to exit

Adding a deprecation message referring to these two new functions would have prevented a lot of head scratching in the two cases above.