bpo-25942: make subprocess more graceful on ^C (GH-5026)

Do not allow receiving a SIGINT to cause the subprocess module to trigger an
immediate SIGKILL of the child process.  SIGINT is normally sent to all child
processes by the OS at the same time already as was the established normal
behavior in 2.7 and 3.2.  This behavior change was introduced during the fix to https://bugs.python.org/issue12494 and is generally surprising to command line
tool users who expect other tools launched in child processes to get their own
SIGINT and do their own cleanup.

In Python 3.3-3.6 subprocess.call and subprocess.run would immediately
SIGKILL the child process upon receiving a SIGINT (which raises a
KeyboardInterrupt).  We now give the child a small amount of time to
exit gracefully before resorting to a SIGKILL.

This is also the case for subprocess.Popen.__exit__ which would
previously block indefinitely waiting for the child to die.  This was
hidden from many users by virtue of subprocess.call and subprocess.run
sending the signal immediately.

Behavior change: subprocess.Popen.__exit__ will not block indefinitely
when the exiting exception is a KeyboardInterrupt.  This is done for
user friendliness as people expect their ^C to actually happen.  This
could cause occasional orphaned Popen objects when not using `call` or
`run` with a child process that hasn't exited.

Refactoring involved: The Popen.wait method deals with the
KeyboardInterrupt second chance, existing platform specific internals
have been renamed to _wait().
Also fixes comment typos.

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