gspawn: Use close_range() if available to close FDs between fork/exec

It’s landed in kernel 5.9: http://lkml.iu.edu/hypermail/linux/kernel/2008.0/02649.html

Note, this is untested because I currently don’t have kernel 5.9. We can fix anything up if it breaks once the new syscall is wrapped in glibc.

Signed-off-by: Philip Withnall pwithnall@endlessos.org