BUG-2295/MAINT-2624 unexpected crash around Content-Range: header processing

Not certain what the source of the short data is with one resident but I'm
going to make these problems retryable as they are transport-related.  Lift
the retry detection into a method that should be reusable by others interested
in determining what is retryable.  Trace output handling on the libcurl debug
callback was attrocious.  Some unsafe length handling on my part was protected
by a second layer of defense.  Made that correct and more useful by logging
actual data sizes during trace.