This fixes a minor problem with fcntl.

get_close_on_exec() uses FD_ISSET() to determine the fd state, but this
is not guaranteed to be either 0 of FD_CLOEXEC.  Make that explicit.

Also, the argument of set_close_on_exec() is being AND'ed with the
literal constant 1.  Make it use an explicit FD_CLOEXEC test.