vcard: Fix whitespace handling in line cont's

Previously, multiple whitespace characters at the start of a
continuation line would all be dropped, instead of only the first one.

Also,
 - restrict line continuation characters to SPACE and TAB.

Note that, like before, this identifies the CR (`\r`) character with the
empty string, and thereby notably does not require a CRLF (`\r\n`)
sequence (which is mandated by RFCs 2426, 2425) for line termination
(i.e., `\n` suffices).

Fixes: Bug 1 of issue #9593.

program/lib/Roundcube/rcube_vcard.php

@@ -681,7 +681,7 @@ public static function rfc2425_fold($val)
     private static function vcard_decode($vcard)
     {
         // Perform RFC2425 line unfolding and split lines
-        $vcard = preg_replace(["/\r/", "/\n\\s+/"], '', $vcard);
+        $vcard = str_replace(["\r", "\n ", "\n\t"], '', $vcard);
         $lines = explode("\n", $vcard);
         $result = [];
 
@@ -985,7 +985,7 @@ private static function detect_encoding($string)
         // This will for example exclude photos
 
         // Perform RFC2425 line unfolding and split lines
-        $string = preg_replace(["/\r/", "/\n\\s+/"], '', $string);
+        $string = str_replace(["\r", "\n ", "\n\t"], '', $string);
         $lines = explode("\n", $string);
         $string = '';