This PR ties together pseudoconsoles, user profiles, s4u logons, and
process creation into what is (hopefully) a simple API for various
Tailscale services to obtain Windows access tokens without requiring
knowledge of any Windows passwords. It works both for domain-joined
machines (Kerberos) and non-domain-joined machines. The former case
is fairly straightforward as it is fully documented. OTOH, the latter
case is not documented, though it is fully defined in the C headers in
the Windows SDK. The documentation blanks were filled in by reading
the source code of Microsoft's Win32 port of OpenSSH.
We need to do a bit of acrobatics to make conpty work correctly while
creating a child process with an s4u token; see the doc comments above
startProcessInternal for details.
Updates #12383
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
