ds: new: avoid redundant check, make clobbering fatal

"fileno(undef)" already dies under "use strict", so there's no
need to check for it ourselves.  As far as "fileno($closed_io)"
or "fileno($fake_io)" goes, we'll let epoll_ctl detect the
error, instead.

Our design should make DescriptorMap entries impossible to clobber,
so make it fatal via confess in case it does happen, because
inadvertantly clobbering a FD would be very bad.  While we're at
it, remove a redundant return statement and rely on implicit
returns.

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