CLEANUP: Use the ist() macro whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    char *s;
    @@

    (
    - ist2(s, strlen(s))
    + ist(s)
    |
    - ist2(strdup(s), strlen(s))
    + ist(strdup(s))
    )

Note that this replacement is safe even in the strdup() case, because `ist()`
will not call `strlen()` on a `NULL` pointer. Instead is inserts a length of
`0`, effectively resulting in `IST_NULL`.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 70b3d22..fabc83e 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -1269,7 +1269,7 @@
 				if (addr_to_str(cli_conn->dst, b_tail(params->p), b_room(params->p)) != -1)
 					ptr = b_tail(params->p);
 			if (ptr) {
-				params->srv_name = ist2(ptr, strlen(ptr));
+				params->srv_name = ist(ptr);
 				params->p->data += params->srv_name.len;
 			}
 		}
@@ -1281,7 +1281,7 @@
 			if (addr_to_str(cli_conn->src, b_tail(params->p), b_room(params->p)) != -1)
 				ptr = b_tail(params->p);
 		if (ptr) {
-			params->rem_addr = ist2(ptr, strlen(ptr));
+			params->rem_addr = ist(ptr);
 			params->p->data += params->rem_addr.len;
 		}
 	}