Clarify the send_mtu_probe() function.

This cleans up the PMTU probing function a little bit. It moves the
low-level sending of packets to a separate function, so that the code
reads naturally instead of using a weird for loop with "special
indexes". In addition, comments are moved inside the body of the
function for additional context.

This shouldn't introduce any change of behavior, except for local
discovery which has some minor logic fixes and which now always uses
small packets (16 bytes) because there's no need for a full-length
probe just to try the local network.