Move logic to set file descriptors to a cloexec_fds_from() function in
misc.c so it can be shared by both supervisor-daemon and
start-stop-daemon, and hide the details behind.
Use HAVE_CLOSE_RANGE to tell if system provides a close_range(2)
wrapper, which better explains the purpose.
Add a compat inline which returns -1 if close_range is unavailable.
Natanael Copa