core: namespace: nitpick /dev/ptmx error handling

core: namespace: nitpick /dev/ptmx error handling

If /dev/tty did not exist, or had st_rdev == 0, we ignored it.  And the
same is true for null, zero, full, random, urandom.

If /dev/ptmx did not exist, we treated this as a failure.  If /dev/ptmx had
st_rdev == 0, we ignored it.

This was a very recent change, but there was no reason for ptmx creation
specifically to treat st_rdev == 0 differently from non-existence.  This
confuses me when reading it.

Change the creation of /dev/ptmx so that st_rdev == 0 is
treated as failure.

This still leaves /dev/ptmx as a special case with stricter handling.
However it is consistent with the immediately preceding creation of
/dev/pts/, which is treated as essential, and is directly related to ptmx.

I don't know why we check st_rdev.  But I'd prefer to have only one
unanswered question here, and not to have a second unanswered question
added on top.