Start the loop by finding the end of the option name, the name-value
separator (if any), and the end of the option. Use those pointers to
simplify matching the option name and parsing the option value, and
validate option names and values more strictly. This means that:

• We no longer accept trailing garbage in an option name or value. For instance, we would previously interpret “edns0123” as “edns0” and “timeout:3xyz” as “timeout:3”. This was actually quite lucky because we also failed to recognize the newline at the end of the option line as a whitespace character.

• For options that take a numerical argument, we would previously accept negative values and treat non-numerical arguments as 0, while large numerical arguments would be capped to the option's maximum permitted value. Now, any failure to parse the argument, including overflow, results in the option being left unchanged.

MFC after: 1 week