Fix errors found by PHPStan

Test Plan:
Ran `phpstan analyze --autoload-file=src/__phutil_library_init__.php -c phpstan.neon -l 1 src/` with `phpstan.neon` containing:

  parameters:
      excludes_analyse:
          - */__tests__/*

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17364

src/cache/PhutilAPCKeyValueCache.php

@@ -90,7 +90,7 @@ public function destroyCache() {
     return $this;
   }
 
-  private static function isAPCU() {
+  private static function isAPCu() {
     return function_exists('apcu_fetch');
   }
 

src/cache/PhutilKeyValueCacheStack.php

@@ -68,7 +68,7 @@ public function setCaches(array $caches) {
    * @task    config
    */
   public function setNextTTL($ttl) {
-    $this->ttl = $ttl;
+    $this->nextTTL = $ttl;
     return $this;
   }
 

src/filesystem/Filesystem.php

@@ -384,7 +384,7 @@ public static function changePermissions($path, $umask) {
    * Get the last modified time of a file
    *
    * @param string Path to file
-   * @return Time last modified
+   * @return int Time last modified
    *
    * @task file
    */

src/future/aws/PhutilAWSException.php

@@ -4,6 +4,7 @@ final class PhutilAWSException extends Exception {
 
   private $httpStatus;
   private $requestID;
+  private $params;
 
   public function __construct($http_status, array $params) {
     $this->httpStatus = $http_status;

src/future/exec/PhutilSignalRouter.php

@@ -69,7 +69,8 @@ public function routeSignal($signo) {
       throw new PhutilAggregateException(
         pht(
           'Signal handlers raised exceptions while handling "%s".',
-          phutil_get_signal_name($signo)));
+          phutil_get_signal_name($signo)),
+        $exceptions);
     }
 
     switch ($signo) {

src/future/http/status/HTTPFutureParseResponseStatus.php

@@ -4,6 +4,8 @@ final class HTTPFutureParseResponseStatus extends HTTPFutureResponseStatus {
 
   const ERROR_MALFORMED_RESPONSE = 1;
 
+  private $rawResponse;
+
   public function __construct($code, $raw_response) {
     $this->rawResponse = $raw_response;
     parent::__construct($code);

src/internationalization/PhutilNumber.php

@@ -5,8 +5,9 @@ final class PhutilNumber extends Phobject {
   private $value;
   private $decimals = 0;
 
-  public function __construct($value) {
+  public function __construct($value, $decimals = 0) {
     $this->value = $value;
+    $this->decimals = $decimals;
   }
 
   public function getNumber() {

src/markup/engine/PhutilRemarkupEngine.php

@@ -293,7 +293,7 @@ public function postprocessText(array $dict) {
       $rule->didMarkupText();
     }
 
-    return $this->restoreText(idx($dict, 'output'), $this->isTextMode());
+    return $this->restoreText(idx($dict, 'output'));
   }
 
   public function restoreText($text) {