Replace invalid characters with a symbol instead of returning empty string for the

Replace invalid characters with a symbol instead of returning empty string for the
entire to be encoded string.

Currently, if an email contains such an invalid character, the whole line will be
left out, which is rather confusing. Even more when it's the email subject. This
constant is only available in PHP 5.4 and up.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@14501 7612ce4b-ef26-0410-bec9-ea0150e637f0