optimizations on linux `-tcp` option

1. support -tcp option for linux(EXPERIMENTAL) . To enable tcp transmission, you should enable it on both client and server, supported only under LINUX with iptables, require root privilege.
2. adjust ack/seq follow scheme

discussion on this release:
#696

$ go version
go version go1.12.7 darwin/amd64

$ uname -a
Darwin xtacideMacBook-Pro.local 18.6.0 Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64 x86_64

$ shasum *.tar.gz
972645b7b3679ac262a7432d87a06c785e218808  kcptun-darwin-amd64-20190718.tar.gz
4f11a55741857870f90b5b32a5a50d788e3a2f7b  kcptun-freebsd-amd64-20190718.tar.gz
2d02a7c0c95c1377918715aaa6c6503d4418ff5b  kcptun-linux-386-20190718.tar.gz
28ebaae18d5d83890fb4ab6394a6d0400893cd5c  kcptun-linux-amd64-20190718.tar.gz
58b6b87f80d3255ab18c6fb1c0aad272f6be3591  kcptun-linux-arm5-20190718.tar.gz
cfa3359377c1ed6de5b1e0d42ce76a1a1132340f  kcptun-linux-arm6-20190718.tar.gz
69a02091c11332a27b1362bf8b14af14cad585fc  kcptun-linux-arm64-20190718.tar.gz
6d9a3a52ba4394b1293fd27e64d7367bb3c60783  kcptun-linux-arm7-20190718.tar.gz
f10669e6c8bc153d1a07caa4c85e77ba17966d2c  kcptun-linux-mips-20190718.tar.gz
94e493615294ca81c088bfcff33021d8d167c42a  kcptun-linux-mipsle-20190718.tar.gz
ac8ea8889c5bc405301d1f6d38de04c1ef863a4e  kcptun-windows-386-20190718.tar.gz
932d4a618ab185af4ca9abd076e39d42b1321191  kcptun-windows-amd64-20190718.tar.gz

$ date
2019年 7月18日 星期四 14时24分53秒 CST