Attached is a new version falling back to /proc/self/fd when closefrom(2) is not available (on Unix), working on Linux.
It's indeed much faster than the current approach.
Note that it's only used if _posixsubprocess is not available, because in that case the FD are closed from _posixsubprocess.c:child_exec.
To make it available to _posixsubprocess, I was thinking of putting the closefrom code in a helper function, which would then be called from posix_closefrom and _posixsubprocess.
Is this the proper way to do ?
If yes, what file would be a good recipient for that ?