syscall: refactor environment handling

* Move environment functions to their own files.
  * Rewrite the WASIp2 version of environment variables to be much
    simpler (don't go through C functions).

src/syscall/syscall_nonhosted.go

@@ -87,48 +87,6 @@ const (
 	MAP_ANONYMOUS = MAP_ANON
 )
 
-func runtime_envs() []string
-
-func Getenv(key string) (value string, found bool) {
-	env := runtime_envs()
-	for _, keyval := range env {
-		// Split at '=' character.
-		var k, v string
-		for i := 0; i < len(keyval); i++ {
-			if keyval[i] == '=' {
-				k = keyval[:i]
-				v = keyval[i+1:]
-			}
-		}
-		if k == key {
-			return v, true
-		}
-	}
-	return "", false
-}
-
-func Setenv(key, val string) (err error) {
-	// stub for now
-	return ENOSYS
-}
-
-func Unsetenv(key string) (err error) {
-	// stub for now
-	return ENOSYS
-}
-
-func Clearenv() (err error) {
-	// stub for now
-	return ENOSYS
-}
-
-func Environ() []string {
-	env := runtime_envs()
-	envCopy := make([]string, len(env))
-	copy(envCopy, env)
-	return envCopy
-}
-
 func Open(path string, mode int, perm uint32) (fd int, err error) {
 	return 0, ENOSYS
 }