cmd/ms: read #P/ps2mouse in a char (not in an int)

After the removal of dumb push in crt0 (commit 929014ebca5c738d3854758326de7abfb77c1ef1)
the first byte of the c integer is not zeroed anymore (which is correct).

But since ms.c reads and bit-match a single byte in c, when it's an int some test success/fail
due to the state of the unused bytes.

This makes the mouse turn crazy.

So we turn it into a char, so that bitmasks and tests work as expected.

sys/src/cmd/ms.c

@@ -20,7 +20,7 @@ main(int argc, char *argv[])
 	int shift = 0;
 	int ps2fd;
 	int mousefd;
-	int c;
+	char c;
 	static short msg[4];
 	static int nb;
 	static uint8_t b[] = {0, 1, 4, 5, 2, 3, 6, 7, 0, 1, 2, 3, 2, 3, 6, 7 };