Include "cloexec.h" first, and <unistd.h> before <fcntl.h>.

Include "cloexec.h" first, and <unistd.h> before <fcntl.h>.
(set_cloexec_flag): Use bool for booleans.  All uses changed.
If F_GETFD returns a negative number (not just -1), report a
failure.  Don't use F_SETFD if the flags are already right.
Don't report a failure with F_SETFD unless it returns -1.