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>

types/key/nl.go

@@ -119,7 +119,7 @@ type NLPublic struct {
 // NLPublicFromEd25519Unsafe converts an ed25519 public key into
 // a type of NLPublic.
 //
-// New uses of this function should be avoided, as its possible to
+// New uses of this function should be avoided, as it's possible to
 // accidentally construct an NLPublic from a non network-lock key.
 func NLPublicFromEd25519Unsafe(public ed25519.PublicKey) NLPublic {
 	var out NLPublic