dispatch: do not close stdout and stderr, just flush() instead
Since 3a4c0905f357 "util: always force line buffered stdout when stdout is
a tty", we have two file objects attached to the same STDOUT_FILENO. If one
is closed, the underlying file descriptor is also closed, and writing to
the other file object would crash the Python interpreter in a hard way, at
least on Windows.
So, it seems safer to not close the standard streams. This also matches
the behavior of the default sys.stdout/stderr.close(), which never close
the FILE* streams in C layer.
https://hg.python.org/cpython/file/v2.7.13/Python/sysmodule.c#l1401
% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration: []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration: [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration: [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration: [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration: [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration: [11, 13, 7, 8]