fix(windows): start keymanx64 with ShellExecute

Fixes #5196.

It is complicated to start a `UIAccess=true` process with
`CreateProcess`, so instead we'll use `ShellExecute`. This means passing
a PID instead of a process handle to keymanx64.

I took the opportunity to refactor slightly the main function in
keymanx64, as the failure modes were pretty WET.