Versatile utility function to run external processes from Python

 Versatile utility function to run external processes from Python, with many
 features:
  * runs an external process and waits for it to finish
  * does not deadlock, no matter the process stdout/stderr output
    behaviour
  * returns the exit code, stdout, stderr (separately, both as
    bytestrings), and the total process runtime as a dictionary
  * process can run in a custom environment, either as a modification
    of the current environment or in a new environment from scratch
  * stdin can be fed to the process, the returned dictionary contains
    information how much was read by the process
  * stdout and stderr is printed by default, can be disabled
  * stdout and stderr can be passed to any arbitrary function for live
    processing (separately, both as unicode strings)
  * optionally enforces a time limit on the process