MINOR: mux-h2: add the expire task and its expiration date in "show fd"

Some issues such as #1929 seem to involve a task without timeout but we
can't find the condition to reproduce this in the code. However, not having
this info in the output doesn't help, so this patch adds the task pointer
and its timeout (when the task is non-null). It may be useful to backport
it.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index fc484f8..ac7afcd 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -6594,6 +6594,12 @@
 		      (unsigned int)b_data(tmbuf), b_orig(tmbuf),
 		      (unsigned int)b_head_ofs(tmbuf), (unsigned int)b_size(tmbuf));
 
+	chunk_appendf(msg, " .task=%p", h2c->task);
+	if (h2c->task) {
+		chunk_appendf(msg, " .exp=%s",
+			      h2c->task->expire ? tick_is_expired(h2c->task->expire, now_ms) ? "<PAST>" :
+			      human_time(TICKS_TO_MS(h2c->task->expire - now_ms), TICKS_TO_MS(1000)) : "<NEVER>");
+	}
 
 	return ret;
 }