Minor revisions to dx-{download,upload}-all-* help messages.

Summary: Also format the help strings a bit more nicely in the source code.

Test Plan: Manually examined output of running both scripts with -h.

Reviewers: orodeh, klai

Reviewed By: klai

Differential Revision: https://phabricator.dnanexus.com/D31
Fix line wrapping of option help after turning on RawTextHelpFormatter
Test fix for bash helpers
PTFM-12437: Adding a wrapper for whoami calls
Merge branch 'master' into PTFM-12436-use-whoami-in-tests

Pick up new system_whoami wrapper from upstream.