notify: XDG token is now expected to be prefixed with xdgtoken=

This patch modifies our stdout reader to consume input as we go,
instead of all at once when stdout is closed. This will make it easier
to add support for reading e.g. the daemon assigned notification ID in
the future, and also ensures we see the XDG activation token "as soon
as possible".

Furthermore, to be more future proof, require the XDG activation token
to be prefixed with 'xdgtoken=', and ignore other lines.

Thus, instead of treating *all* of stdout as the XDG activation token,
parse stdout line-by-line, and ignore everything that does not begin
with 'xdgtoken='. Everything (on that line) following 'xdgtoken=' is
treated as the activation token.

notify.h

@@ -23,18 +23,30 @@ enum notify_urgency {
 };
 
 struct notification {
+    /*
+     * Set by caller of notify_notify()
+     */
     char *id;
     char *title;
     char *body;
     char *icon;
-    char *xdg_token;
+
     enum notify_when when;
     enum notify_urgency urgency;
     bool focus;
     bool report;
 
-    pid_t pid;
-    int stdout_fd;
+    /*
+     * Used internally by notify
+     */
+
+    char *xdg_token;       /* XDG activation token, from daemon */
+
+    pid_t pid;             /* Notifier command PID */
+    int stdout_fd;         /* Notifier command's stdout */
+
+    char *stdout;          /* Data we've reado from command's stdout */
+    size_t stdout_sz;
 };
 
 bool notify_notify(const struct terminal *term, struct notification *notif);