Fix mmap check on Mac OS.

On Mac OS the system reports a nonzero size when you stat stdin. We
assumed this would always be 0 for files that cannot be mmap'd. This
caused "dx upload -" to fail on Macs.

Fix this check by replacing it with checks on the stat mode.
This is a simplified version of b15ac9fb7b9dab50da6f264889bc530ac18d0e51 (see comment in the commit).
Fixes for dx get applet-xxxx

* Fix retrieval of properties, which was broken before
* Retrieve details
* Do not change applet semantics (in particular, inputSpec/outputSpec)
  when doing cleanup
Changelog and dxR version update for v0.80.1