traceroute: cleanup and fixes for packet size calculations

traceroute: cleanup and fixes for packet size calculations

Remove FEATURE_TRACEROUTE_SOURCE_ROUTE: it's off by default, and
source routing is not used in real world.

Tested that "traceroute -n ::1 100" and "traceroute -n 127.0.0.1 100"
both send 100 byte IP packets (this matches what traceroute on Fedora
Rawhide is doing).

function                                             old     new   delta
common_traceroute_main                              3731    3738      +7

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>