pathmtu/pathmtu.{c,1.in}: New algorithms for path-MTU discovery.

pathmtu/pathmtu.{c,1.in}: New algorithms for path-MTU discovery.

Implement path-MTU discovery using raw sockets rather than Linux's fancy
socket options.  This should improve portability to other systems.

This involves splitting the logic which chooses probe sizes and
determines the resulting MTU from the underlying machinery which
manufactures packets and collects results.