Optimize the :mod:`subprocess` module on FreeBSD using ``closefrom()``.
A single ``close(fd)`` syscall is cheap, but when ``sysconf(_SC_OPEN_MAX)`` is
high, the loop calling ``close(fd)`` on each file descriptor can take several
milliseconds.

The workaround on FreeBSD to improve performance was to load and mount the
fdescfs kernel module, but this is not enabled by default.

Initial patch by Ed Maste (emaste), Conrad Meyer (cem), Kyle Evans (kevans) and
Kubilay Kocak (koobs):
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=242274
