Remove the optional 0x prefix from the keyID

It is possible to invoke pinentry with different keyID formats via the
--keyid-format flag, for instance. This previously broke the length
validation for the keyID which should be either 8/short or 16/long bytes
without the 0x prefix.

Previous issue could be reproduced via:

echo 1234 | /usr/local/bin/gpg --default-key $TESTKEY --keyid-format 0xshort -as -

Extends the implementation from #16.

main.go

@@ -247,7 +247,12 @@ func GetPIN(authFn AuthFunc, promptFn PromptFunc, logger *log.Logger) GetPinFunc
 
 		matches = keyIDRegex.FindStringSubmatch(s.Desc)
 		keyID := matches[1]
-		if len(keyID) != 8 && len(keyID) != 16 && len(keyID) != 18 {
+
+		// Drop the optional 0x prefix from keyID (--keyid-format)
+		// https://www.gnupg.org/documentation/manuals/gnupg/GPG-Configuration-Options.html
+		keyID = strings.TrimPrefix(keyID, "0x")
+
+		if len(keyID) != 8 && len(keyID) != 16 {
 			logger.Printf("Invalid keyID: %s", keyID)
 			return "", assuanError(fmt.Errorf("invalid keyID: %s", keyID))
 		}