MINOR: mux-h2: report the mbuf's head and tail in "show fd"

It's useful to know how the mbuf spans over the whole area and to have
access to the first and last ones, so let's dump just this.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 1d64d26..e727549 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -5620,7 +5620,7 @@
 	int send_cnt = 0;
 	int tree_cnt = 0;
 	int orph_cnt = 0;
-	struct buffer *tmbuf;
+	struct buffer *hmbuf, *tmbuf;
 
 	if (!h2c)
 		return;
@@ -5641,16 +5641,21 @@
 		node = eb32_next(node);
 	}
 
+	hmbuf = br_head(h2c->mbuf);
 	tmbuf = br_tail(h2c->mbuf);
 	chunk_appendf(msg, " h2c.st0=%d .err=%d .maxid=%d .lastid=%d .flg=0x%04x"
 		      " .nbst=%u .nbcs=%u .fctl_cnt=%d .send_cnt=%d .tree_cnt=%d"
-		      " .orph_cnt=%d .sub=%d .dsi=%d .dbuf=%u@%p+%u/%u .msi=%d .mbuf=%u@%p+%u/%u",
+		      " .orph_cnt=%d .sub=%d .dsi=%d .dbuf=%u@%p+%u/%u .msi=%d"
+		      " .mbuf=[%u..%u|%u],h=[%u@%p+%u/%u],t=[%u@%p+%u/%u]",
 		      h2c->st0, h2c->errcode, h2c->max_id, h2c->last_sid, h2c->flags,
 		      h2c->nb_streams, h2c->nb_cs, fctl_cnt, send_cnt, tree_cnt, orph_cnt,
 		      h2c->wait_event.events, h2c->dsi,
 		      (unsigned int)b_data(&h2c->dbuf), b_orig(&h2c->dbuf),
 		      (unsigned int)b_head_ofs(&h2c->dbuf), (unsigned int)b_size(&h2c->dbuf),
 		      h2c->msi,
+		      br_head_idx(h2c->mbuf), br_tail_idx(h2c->mbuf), br_size(h2c->mbuf),
+		      (unsigned int)b_data(hmbuf), b_orig(hmbuf),
+		      (unsigned int)b_head_ofs(hmbuf), (unsigned int)b_size(hmbuf),
 		      (unsigned int)b_data(tmbuf), b_orig(tmbuf),
 		      (unsigned int)b_head_ofs(tmbuf), (unsigned int)b_size(tmbuf));