MINOR: trace: accept trace spec right after "-dt" on the command line

MINOR: trace: accept trace spec right after "-dt" on the command line

I continue to mistakenly set the traces using "-dtXXX" and to have to
refer to the doc to figure that it requires a separate argument and
differs from some other options. Worse, "-dthelp" doesn't say anything
and silently ignores the argument.

Let's make the parser take whatever follows "-dt" as the argument if
present, otherwise take the next one (as it currently does). Doing
this even allows to simplify the code, and is easier to figure the
syntax since "-dthelp" now works.