syscall: SFD_NONBLOCK can be a constant, again

syscall: SFD_NONBLOCK can be a constant, again

Since Perl exposes O_NONBLOCK as a constant, we can safely make
SFD_NONBLOCK a constant, too.  This is not the case for
SFD_CLOEXEC, since O_CLOEXEC is not exposed by Perl despite
being used internally in the interpreter.