windows: replace all StringToUTF16 calls with UTF16FromString

`StringToUTF16` is deprecated and will panic if given an "invalid"
string (in particular, one that has a null byte in it).  The replacement
function is `UTF16FromString`, and it returns an error if there was
a problem.

This change replaces all uses of `StringToUTF16` with `UTF16FromString`.
The `service` struct now no longer stores a `string` name but rather
a `*uint16` pointer to the name.

It should not be possible to panic due to UTF16 string conversion
at this point.

Fixes golang/go#73006

Change-Id: Idce9cdbb4651fef8481f0cad19b5df0314fd4277
Reviewed-on: https://go-review.googlesource.com/c/sys/+/659936
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

windows/svc/mgr/service.go

@@ -37,7 +37,11 @@ func (s *Service) Start(args ...string) error {
 	if len(args) > 0 {
 		vs := make([]*uint16, len(args))
 		for i := range vs {
-			vs[i] = syscall.StringToUTF16Ptr(args[i])
+			argPointer, err := syscall.UTF16PtrFromString(args[i])
+			if err != nil {
+				return err
+			}
+			vs[i] = argPointer
 		}
 		p = &vs[0]
 	}