@ZaphodB fix(@security): S/MIME actions put the client's part id into IMAP FETCH as is

DoSMimeDecryptMessage and DoSMimeVerifyMessage built BODY.PEEK[<partId>]
(and [<partId>.MIME]) from the partId / sigPartId parameters without checking
them, so a value could close the bracket or the line and add IMAP commands of
its own. They run in the user's own IMAP session, so this is not cross-account,
but nothing in that session should be steerable by a request parameter.

Read both through smimePartIdParam(), which accepts only an IMAP section number
(1, 1.2.10) or TEXT and refuses anything else.

test/smime-partid.php checks valid ids pass and injection attempts, zero or
leading-zero sections, lowercase and other section keywords are refused.

test/smime-partid.php

@@ -0,0 +1,37 @@
+<?php
+
+// Run with: php test/smime-partid.php
+
+// Enable SnappyMail Api and include index file
+$_ENV['SNAPPYMAIL_INCLUDE_AS_API'] = true;
+require __DIR__ . '/../index.php';
+
+function check(bool $condition, string $message): void
+{
+	if (!$condition) {
+		throw new RuntimeException($message);
+	}
+}
+
+$actions = (new ReflectionClass(\RainLoop\Actions::class))->newInstanceWithoutConstructor();
+$params = new ReflectionProperty(\RainLoop\Actions::class, 'aCurrentActionParams');
+$method = new ReflectionMethod(\RainLoop\Actions::class, 'smimePartIdParam');
+$read = function (string $value) use ($actions, $params, $method) : ?string {
+	$params->setValue($actions, ['partId' => $value]);
+	try {
+		return $method->invoke($actions, 'partId');
+	} catch (\RainLoop\Exceptions\ClientException $e) {
+		return null;
+	}
+};
+
+foreach (['', '1', '2', '1.2', '1.2.10', 'TEXT'] as $ok) {
+	check($ok === $read($ok), "valid part id '{$ok}' refused");
+}
+foreach (["1]\r\nA1 LOGOUT\r\n", '1] BODY[HEADER', "1\r\nA1 LOGOUT", '1 2', '0', '1.0', '01', '1.', '.1', 'text', 'HEADER'] as $bad) {
+	check(null === $read($bad), 'part id '.json_encode($bad).' accepted');
+}
+// Surrounding whitespace is trimmed away, and the trimmed value is what is used.
+check('1' === $read("1\n"), 'trailing newline not trimmed');
+
+echo "smime-partid: ok\n";