Feature: Add hook "prior process start"

[htgoebel]

Please a hook or some other means to run just prior to starting the external process. This would allow to e.g. set environment variables valid for the new process.

Use-case:
For every process I need to set $HOME to the specific test's tmp_path. I'm testing a CIFF interface to some C-library which uses sqlite to store data in $HOME/…. The C-library does not offer any API to "release" the database, but keeps some db-connections open until the operating system purges the library. (Even worse: The C-lib lets sqlite do the job ob purging/cleanup.) So each test needs to be run in a separate process with a separate $HOME.