MEDIUM: correctly support EAGAIN on send()

Till now, send() returning EAGAIN would be considered as an error,
which is wrong despite being done only after a poll. Also, this can
allow to do it without polling, despite not bringing any benefit here
since we're using select().
MEDIUM: add -D to set a random distance on the outgoing TTL value

The principle is to have outgoing packets leave with a TTL between
64 and 127 solely based on their source /24 network when -S is used.