run-command: add close_fd_above_stderr option

run-command: add close_fd_above_stderr option

Add a close_fd_above_stderr flag to struct child_process.  When set,
the child closes file descriptors 3 and above between fork and exec
(skipping the child-notifier pipe), capped at sysconf(_SC_OPEN_MAX)
or 4096, whichever is smaller.  This prevents the child from
inheriting pipe endpoints or other descriptors from the parent
environment (e.g., the test harness).

Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>