Make local mksyscall_windows.go forks less forked

At this point points of difference between the three local
mksyscall_windows.go implementations and
golang.org/x/sys/windows/mkwinsyscall v0.0.0-20210104204734-6f8348627aad
are:
- pkg/etw: Deduplicates imported functions due to multiple calling APIs
- pkg/security and vhd: Forced UTF-16 mode, not checking A/W suffix. I
  also removed cosmetic differences between these two implementations.

`go generate` reports no changes with these updates.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

vhd/mksyscall_windows.go

@@ -599,7 +599,7 @@ func (p *Fn) MaybeAbsent() string {
 
 // IsUTF16 is true, if f is W (utf16) function. It is false
 // for all A (ascii) functions.
-func (f *Fn) IsUTF16() bool {
+func (_ *Fn) IsUTF16() bool {
 	return true
 }