Feature/pmtu discovery

[acooks]

Here is a potential implementation of path MTU discovery. It has recently been mentioned in issue #54 and #71

Some test results here:

https://github.com/acooks/icmplib/wiki

The results show that interfaces conform to the Robustness principle in that they may accept packets somewhat larger than the configured MTU, but won't send anything larger than the configured MTU.

Characterizing that asymmetric behavior was what motivated me to implement the pmtu discovery.