Fix build failure on illumos

Previously v2ray can not be built on illumos due to following reasons:
1. missing build tags in transport/internet/sockopt_other.go
2. many definitions in syscall does not exist on illumos
This commit addresses these problems by adding missing build tags, and
updates those missing syscall deps on illumos to use x/sys/unix.

common/buf/readv_posix.go

@@ -1,5 +1,6 @@
 // +build !windows
 // +build !wasm
+// +build !illumos
 
 package buf