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_libc.go

@@ -238,58 +238,6 @@ func Wait4(pid int, wstatus *WaitStatus, options int, rusage uintptr) (wpid int,
 	return 0, ENOSYS // TODO
 }
 
-func Getenv(key string) (value string, found bool) {
-	data := cstring(key)
-	raw := libc_getenv(&data[0])
-	if raw == nil {
-		return "", false
-	}
-
-	ptr := uintptr(unsafe.Pointer(raw))
-	for size := uintptr(0); ; size++ {
-		v := *(*byte)(unsafe.Pointer(ptr))
-		if v == 0 {
-			src := *(*[]byte)(unsafe.Pointer(&sliceHeader{buf: raw, len: size, cap: size}))
-			return string(src), true
-		}
-		ptr += unsafe.Sizeof(byte(0))
-	}
-}
-
-func Setenv(key, val string) (err error) {
-	if len(key) == 0 {
-		return EINVAL
-	}
-	for i := 0; i < len(key); i++ {
-		if key[i] == '=' || key[i] == 0 {
-			return EINVAL
-		}
-	}
-	for i := 0; i < len(val); i++ {
-		if val[i] == 0 {
-			return EINVAL
-		}
-	}
-	runtimeSetenv(key, val)
-	return
-}
-
-func Unsetenv(key string) (err error) {
-	runtimeUnsetenv(key)
-	return
-}
-
-func Clearenv() {
-	for _, s := range Environ() {
-		for j := 0; j < len(s); j++ {
-			if s[j] == '=' {
-				Unsetenv(s[0:j])
-				break
-			}
-		}
-	}
-}
-
 func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
 	addr := libc_mmap(nil, uintptr(length), int32(prot), int32(flags), int32(fd), uintptr(offset))
 	if addr == unsafe.Pointer(^uintptr(0)) {