all: fix typos in comments

Fix its/it's, who's/whose, wether/whether, missing apostrophes
in contractions, and other misspellings across the codebase.

Updates #cleanup

Change-Id: I20453b81a7aceaa14ea2a551abba08a2e7f0a1d8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -429,7 +429,7 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, actor ipnauth.Actor) (o
 	if len(s.activeReqs) == 1 {
 		if envknob.GOOS() == "windows" && !actor.IsLocalSystem() {
 			// Tell the LocalBackend about the identity we're now running as,
-			// unless its the SYSTEM user. That user is not a real account and
+			// unless it's the SYSTEM user. That user is not a real account and
 			// doesn't have a home directory.
 			lb.SetCurrentUser(actor)
 		}