Display original command line argv via no-op flag.

Improves 679b8192 to make it simpler to
ignore mosh-client from OS X Terminal.app's "Ask before closing" process
list. (See: http://superuser.com/a/254647)

Moreover, the command displayed from ps is a completely functional one,
which was not the case in the previous approach, that mangles `argv[0]`.

Fixes #742.