MINOR: buffer: merge b{i,o}_contig_space()

These ones were merged into a single b_contig_space() that covers both
(the bo_ case was a simplified version of the other one). The function
doesn't use ->i nor ->o anymore.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index e2f96c7..0284279 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2989,7 +2989,7 @@
 
 	while (1) {
 		outbuf.str  = b_tail(h2c->mbuf);
-		outbuf.size = bo_contig_space(h2c->mbuf);
+		outbuf.size = b_contig_space(h2c->mbuf);
 		outbuf.len = 0;
 
 		if (outbuf.size >= 9 || !b_space_wraps(h2c->mbuf))
@@ -3147,7 +3147,7 @@
 
 	while (1) {
 		outbuf.str  = b_tail(h2c->mbuf);
-		outbuf.size = bo_contig_space(h2c->mbuf);
+		outbuf.size = b_contig_space(h2c->mbuf);
 		outbuf.len = 0;
 
 		if (outbuf.size >= 9 || !b_space_wraps(h2c->mbuf))