vchoi_fork

Added support for vector I/O to the VFD layer, and associated test code
in test/vfd.c.  Note that this test uses only sec2 and stdio at present.
Since no VFDs support vector I/O at present, the vector I/O VFD calls
are translated to regular read/write calls in the VFD layer.

Tested serial and parallel, debug and production on charis, parallel
debug on jelly.  Some seemingly un-related issues, but after consultation
with Richard, I'm proceeding with the checkin.