traceroute: graceful recover after networking errors with as_server

Currentlu, the traceroute[6] utility does not check
for possible networking errors while talking to as_server
in case of "traceroute -a". Meantime, there is a common case
when trace is long and it has many non-responding hops
(shown as '* * *'), so as_server aborts our connection,
hence no AS numbers shown for the rest of trace.

Somewhat artifical way to reproduce the problem:

traceroute to 57.144.244.1 (57.144.244.1), 80 hops max, 48 byte packets
5 [AS174] be2950.ccr42.fra05.atlas.cogentco.com (154.54.72.42) 74.277 ms 9.605 ms 9.599 ms
6 [AS174] 149.11.228.19 (149.11.228.19) 9.506 ms 9.466 ms 9.420 ms
7 [AS33182] po172.asw02.fra5.tfbnw.net (129.134.108.146) 9.725 ms 9.874 ms 9.696 ms
8 [AS32934] psw04.fra5.tfbnw.net (157.240.59.85) 8.718 ms 8.691 ms 8.618 ms
9 * * *
10 [AS0] edge-star-mini-shv-01-fra5.facebook.com (57.144.244.1) 9.747 ms 9.815 ms 9.699 ms

Note what due to increased timeout (-w10) we get [AS0] for 10th hop
because as_server closed our connection. Same problem occurs
with default -w3 when there are lots of '* * *' hops in a trace.

Fix it with some additional error checking and a reconnection.

This patch was in my use for many years and after traceroute(8) moved
from contrib/ to usr.sbin/ it's nice to have it in the tree.

MFC after: 2 weeks