Make all popen(3) file descriptors O_CLOEXEC.

POSIX says "The popen() function shall ensure that any streams from
previous popen() calls that remain open in the parent process are closed
in the new child process". It doesn't appear to disallow all popen(3) file
descriptors from being O_CLOEXEC, and it's not obvious why anyone would want
them inherited. Let's see if we can make the stricter guarantee...

Bug: N/A
Test: ran tests
Change-Id: I2c85170d730b211637afb8ba10df150ca3237262