cmd/tailscaled, util/winutil: log our registry keys during tailscaled startup

In order to improve our ability to understand the state of policies and
registry settings when troubleshooting, we enumerate all values in all subkeys.
x/sys/windows does not already offer this, so we need to call RegEnumValue
directly.

For now we're just logging this during startup, however in a future PR I plan to
also trigger this code during a bugreport. I also want to log more than just
registry.

Fixes #8141

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

util/winutil/zsyscall_windows.go

@@ -7,6 +7,7 @@ import (
 	"unsafe"
 
 	"golang.org/x/sys/windows"
+	"golang.org/x/sys/windows/registry"
 )
 
 var _ unsafe.Pointer
@@ -41,6 +42,7 @@ var (
 	modadvapi32 = windows.NewLazySystemDLL("advapi32.dll")
 
 	procQueryServiceConfig2W = modadvapi32.NewProc("QueryServiceConfig2W")
+	procRegEnumValueW        = modadvapi32.NewProc("RegEnumValueW")
 )
 
 func queryServiceConfig2(hService windows.Handle, infoLevel uint32, buf *byte, bufLen uint32, bytesNeeded *uint32) (err error) {
@@ -50,3 +52,11 @@ func queryServiceConfig2(hService windows.Handle, infoLevel uint32, buf *byte, b
 	}
 	return
 }
+
+func regEnumValue(key registry.Key, index uint32, valueName *uint16, valueNameLen *uint32, reserved *uint32, valueType *uint32, pData *byte, cbData *uint32) (ret error) {
+	r0, _, _ := syscall.Syscall9(procRegEnumValueW.Addr(), 8, uintptr(key), uintptr(index), uintptr(unsafe.Pointer(valueName)), uintptr(unsafe.Pointer(valueNameLen)), uintptr(unsafe.Pointer(reserved)), uintptr(unsafe.Pointer(valueType)), uintptr(unsafe.Pointer(pData)), uintptr(unsafe.Pointer(cbData)), 0)
+	if r0 != 0 {
+		ret = syscall.Errno(r0)
+	}
+	return
+}