windows: implement the environment functions by wrapping syscall

Make windows implementation similar to unix.

Updates golang/go#10803 (0intro will close the issue)

Change-Id: I6f6a7b1c84f54b1ec92f0346a9998df34235c71a
Reviewed-on: https://go-review.googlesource.com/10077Reviewed-by: Rob Pike <r@golang.org>

windows/env_windows.go

@@ -6,75 +6,20 @@
 
 package windows
 
-import (
-	"unicode/utf16"
-	"unsafe"
-)
+import "syscall"
 
 func Getenv(key string) (value string, found bool) {
-	keyp, err := UTF16PtrFromString(key)
-	if err != nil {
-		return "", false
-	}
-	n := uint32(100)
-	for {
-		b := make([]uint16, n)
-		n, err = GetEnvironmentVariable(keyp, &b[0], uint32(len(b)))
-		if n == 0 && err == ERROR_ENVVAR_NOT_FOUND {
-			return "", false
-		}
-		if n <= uint32(len(b)) {
-			return string(utf16.Decode(b[:n])), true
-		}
-	}
+	return syscall.Getenv(key)
 }
 
 func Setenv(key, value string) error {
-	v, err := UTF16PtrFromString(value)
-	if err != nil {
-		return err
-	}
-	keyp, err := UTF16PtrFromString(key)
-	if err != nil {
-		return err
-	}
-	e := SetEnvironmentVariable(keyp, v)
-	if e != nil {
-		return e
-	}
-	return nil
+	return syscall.Setenv(key, value)
 }
 
 func Clearenv() {
-	for _, s := range Environ() {
-		// Environment variables can begin with =
-		// so start looking for the separator = at j=1.
-		// http://blogs.msdn.com/b/oldnewthing/archive/2010/05/06/10008132.aspx
-		for j := 1; j < len(s); j++ {
-			if s[j] == '=' {
-				Setenv(s[0:j], "")
-				break
-			}
-		}
-	}
+	syscall.Clearenv()
 }
 
 func Environ() []string {
-	s, e := GetEnvironmentStrings()
-	if e != nil {
-		return nil
-	}
-	defer FreeEnvironmentStrings(s)
-	r := make([]string, 0, 50) // Empty with room to grow.
-	for from, i, p := 0, 0, (*[1 << 24]uint16)(unsafe.Pointer(s)); true; i++ {
-		if p[i] == 0 {
-			// empty string marks the end
-			if i <= from {
-				break
-			}
-			r = append(r, string(utf16.Decode(p[from:i])))
-			from = i + 1
-		}
-	}
-	return r
+	return syscall.Environ()
 }