optimizations on -tcp option for linux

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.

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
023aea9a77a06c36413f78f3ff96a153da1f28b5  kcptun-linux-amd64-20190716.tar.gz
069cef75d5522f127f0f7d979ccb9538137ada66  kcptun-darwin-amd64-20190716.tar.gz
6b494a14e44bea9f07c0d8f5793ca7ff7efea297  kcptun-windows-amd64-20190716.tar.gz
8615af8b73d7d9860de03c601befdc08e312f6e0  kcptun-freebsd-amd64-20190716.tar.gz
b939aec4cfe6f1af5cb3d0d627fce79026a1e3cf  kcptun-linux-386-20190716.tar.gz
df97d2f77cdd605f732a403021ad698cbba295f3  kcptun-windows-386-20190716.tar.gz
1e462e4373d4e8d4d4354b22a0d269a2de7720b8  kcptun-linux-arm5-20190716.tar.gz
848a1fbcfeda301ae66966e42124385252dccb7f  kcptun-linux-arm6-20190716.tar.gz
12ad0ee691926835c80b73b7f836ca70ef806d3e  kcptun-linux-arm7-20190716.tar.gz
5ad6e08b1b99f934fe257c5edec41f6b68031af7  kcptun-linux-arm64-20190716.tar.gz
4ddb9d41e60ccdd8aa8fdf1496a30470d36af6d9  kcptun-linux-mipsle-20190716.tar.gz
127c1956a885581d082a8f69eecb146c62b6cd2a  kcptun-linux-mips-20190716.tar.gz

$ date
2019年 7月16日 星期二 17时00分26秒 CST