increase PassCmd output buffer even more

apparently, some XOAUTH2 tokens are at 2.4K already, so make it 8K to be
*really* safe for a while.

REFMAIL: <20200716000515.GA2111668@lysator.liu.se>

src/drv_imap.c

@@ -1893,7 +1893,7 @@ ensure_password( imap_server_conf_t *srvc )
 	if (cmd) {
 		FILE *fp;
 		int ret;
-		char buffer[2048];  // Hopefully more than enough room for XOAUTH2, etc. tokens
+		char buffer[8192];  // Hopefully more than enough room for XOAUTH2, etc. tokens
 
 		if (*cmd == '+') {
 			flushn();