main/musl: backport patch for popen leaking pipes

Without this patch, musl leaks file descriptors to the child in applications which use multiple concurrent popen pipes but don't request close-on-exec. This can cause deadlocks with some application (e.g. mblaze). Since this patch fixes real bugs in real application (see !19262 (merged)) I would suggest backporting it since there will likely be no new musl release before Alpine release 3.15 is due.

The 0001-remove-spurious-lock-in-popen.patch has also been backported to make the patch apply cleanly.

See also:

• https://github.com/leahneukirchen/mblaze/issues/203
• https://www.openwall.com/lists/musl/2021/03/14/1