fix 'too man open files'
$ go version
go version go1.10 darwin/amd64
03f27ec
xtaci/kcp-go@42bc1df
xtaci/smux@e28719e
bd6fa3a725d62a5ad78f948fd40ab43f9876f908 kcptun-linux-amd64-20180316.tar.gz
caa80772285f68bd8653a8d2ce7c651b753d75b5 kcptun-linux-386-20180316.tar.gz
da968460fda2117d7148093f9e6f763beadc7185 kcptun-darwin-amd64-20180316.tar.gz
6107a98b318bf4bf476175101cd009bf530614f8 kcptun-darwin-386-20180316.tar.gz
81b0ce6ad3fb955264a0a7a74268b0686b69475e kcptun-windows-amd64-20180316.tar.gz
d5f60ca1d866439938fa3ea4002e2099cc8a43d2 kcptun-windows-386-20180316.tar.gz
0c809f57586bb954b1426de6896ef3741b3aa481 kcptun-freebsd-amd64-20180316.tar.gz
fb5217c37d3029ae003dca876b92d6bfd82ad34f kcptun-freebsd-386-20180316.tar.gz
ca28edd9f7a8a09bff564a8782b7115f0d6e02ff kcptun-linux-arm-20180316.tar.gz
a1a47da98ff2744a418b2c68646601ed37d47b6d kcptun-linux-mipsle-20180316.tar.gz
99ef72cbcd03222d3bd3344e72188edbe1fc4a64 kcptun-linux-mips-20180316.tar.gz