The earlier commit to read from stdin was full of problems, and
this corrects them.
- The mode bits should have been set to satisify S_ISREG(); we
forgot to the S_IFREG bits and hardcoded 0644;
- We did not give escape hatch to name a path whose name is
really "-". Allow users to say "./-" for that;
- Use of xread() was not prepared to see short read (e.g. reading
from tty) nor handing read errors.