do not link statically to system libs (glibc, threads, etc)

ocamlopt reorders -ccopt so we have to collect all extra libs into
single list and only then wrap it with linker flags -Bstatic .. -Bdynamic
DC: enable porttest
use custom protocol independent connection test
simplify Printexc2 even further
use Printexc2.to_string everywhere
BT: better diagnostics of wrong udp tracker reply
configure: detect git revesion
BT/DHT: update public router addresses

bt_dht_node: specify bootstrap peers from command-line