OS support: fix writeable client vs IgnoreClient behavior

When ResetCurrentRequest is called, or IgnoreClient is called when a
client has input pending, IgnoredClientsWithInput will be set.  However,
a subsequent IgnoreClient request will clear the client fd from that fd
set, potentially causing the client to hang.

So add an Ignore/Attend count, and only apply the ignore logic on the
first ignore and the attend logic on the last attend.  This is
consistent with the comments for these functions; callers must pair
them.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=27035.

Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>

include/dixstruct.h

@@ -98,6 +98,7 @@ typedef struct _Client {
     int         clientGone;
     int         noClientException;	/* this client died or needs to be
 					 * killed */
+    int         ignoreCount;		/* count for Attend/IgnoreClient */
     SaveSetElt	*saveSet;
     int         numSaved;
     int         (**requestVector) (