Allow filtered streams to be casted as fd for select

This removes the artificial limitation that is not necessary. The fact
that some streams can have some data buffered is not a problem because
the similar situation is already present for OpenSSL streams where
OpenSSL can internally buffer data for the unprocessed part of the
record.

Closes GH-20540

UPGRADING

@@ -49,6 +49,7 @@ PHP 8.6 UPGRADE NOTES
   . Added stream socket context options so_keepalive, tcp_keepidle,
     tcp_keepintvl and tcp_keepcnt that allow setting socket keepalive
     options.
+  . Allowed casting casting filtered streams as file descriptor for select.
 
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 3. Changes in SAPI modules