DoCommand: properly save the last partial line

Previously, the code computed the last partial properly (it called it
the "tail" here in this routine, but called it "partial" elsewhere).
But it never assigned that value back into $$partial so that the
caller would get the actual value of the partial.
DoCommand: fix minor (harmless) typo
Funclets: Add &cmd_stdout() and &cmd_stderr() funclets

These two funclets return the stdout/stderr from the last command that
was run.  This is most helpful in pass/fail criteria for test runs,
where the stdout/stderr that is returned is that from the last test
that was run.
Merge pull request #408 from jsquyres/topic/daves-trail-of-tears

Various fixes