This fixes problem of creation the garbage of zombie processes on POSIX
systems. This also makes behaviour of `popen2` identical in Windows,
namely, all resources are freed only when waiting, giving at the same
time an ability to observe the exit code afterwards.
* Add function that checks whether a file is a terminal device.
* support isTTY function for NodeJS backend.
* don't accidentally interpret 'false' string as truthy number
* less code duplication.
Pragma once is supported by all compilers for the last ten years.
Better use it instead of include guards (which use different styles
in different files).
