Implement a "read-only" version of nextpart()

Implement a "read-only" version of nextpart()

The system test helper function nextpart() always updates the "lines
read so far" marker ("<file>.prev") when it is called, which somewhat
limits its flexibility.  Add two new helper functions, nextpartpeek()
and nextpartreset(), so that certain parts of log files can be easily
examined more than once.  Add some documentation to help understand the
purpose of each function in the nextpart*() family.

(cherry picked from commit facb68b22e36b2959ccbc1c3b1680c1814988927)