TODO

* move stdout/stderr fds to pipe_writer

* unify process::read between Linux and Windows; use non-blocking
  polling in both places and handle Ctrl+C
* make Ctrl+C abandon read/write to/from a process
* prevent Ctrl+C from being passed to the child process

* implement a high-level popen() call/factor function with parameters:
  stdin, stdout, stderr; enable string-based input, output to /dev/null
  (Linux) or nul (Windows); redirecting stderr to stdout

* make partial UTF8 handling optional in process_read(); maybe someone
  is reading bytes and not textual output?
* handle differing new-line characters

* see how tools::pskill can be used our own process_send_signal