Added evhtp_accept_socket

Based on https://github.com/ellzey/libevhtp/pull/126 @simonas added a function
to setup the listener on an already bound socket.

The only problem I had was the duplicate code; so I fixed it in this manner
(and it flows quite well)

evhtp_bind_socket parses the data to which the server will listen on, then
creates a struct sockaddr.

evhtp_bind_socket then calls evhtp_bind_sockaddr, which creates the socket,
sets some options, and calls bind.

evhtp_bind_sockaddr will then call evhtp_accept_socket with the newly created
socket. evhtp_accept_socket will then create the listener context using only
that file descriptor (and the evhtp_t structure).

The result is clean and exactly what @simonas wanted without the code
duplication.

evhtp.c

@@ -3179,53 +3179,40 @@ evhtp_unbind_socket(evhtp_t * htp) {
 }
 
 int
-evhtp_bind_sockaddr(evhtp_t * htp, struct sockaddr * sa, size_t sin_len, int backlog) {
-#ifndef WIN32
-    signal(SIGPIPE, SIG_IGN);
-#endif
-    evutil_socket_t fd;
-    int             on = 1;
-
-    fd = socket(sa->sa_family, SOCK_STREAM, 0);
-    evhtp_errno_assert(fd != -1);
-
-    evutil_make_socket_closeonexec(fd);
-    evutil_make_socket_nonblocking(fd);
-
-    setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, (void *)&on, sizeof(on));
-    setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (void *)&on, sizeof(on));
+evhtp_accept_socket(evhtp_t * htp, evutil_socket_t sock, int backlog) {
+    int on = 1;
 
-    if (sa->sa_family == AF_INET6) {
-        int rc;
+    evhtp_assert(htp != NULL);
 
-        rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on));
-        evhtp_errno_assert(rc != -1);
+    if (sock == -1) {
+        return -1;
     }
 
 #if defined SO_REUSEPORT
     if (htp->enable_reuseport) {
-        setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, (void *)&on, sizeof(on));
+        setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (void *)&on, sizeof(on));
     }
 #endif
 
 #if defined TCP_NODELAY
     if (htp->enable_nodelay == 1) {
-        setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (void *)&on, sizeof(on));
+        setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, (void *)&on, sizeof(on));
     }
 #endif
 
 #if defined TCP_DEFER_ACCEPT
     if (htp->enable_defer_accept == 1) {
-        setsockopt(fd, IPPROTO_TCP, TCP_DEFER_ACCEPT, (void *)&on, sizeof(on));
+        setsockopt(sock, IPPROTO_TCP, TCP_DEFER_ACCEPT, (void *)&on, sizeof(on));
     }
 #endif
 
-    evhtp_errno_assert(bind(fd, sa, sin_len) != -1);
-
     htp->server = evconnlistener_new(htp->evbase, _evhtp_accept_cb, htp,
                                      LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE,
-                                     backlog, fd);
-    evhtp_errno_assert(htp->server != NULL);
+                                     backlog, sock);
+
+    if (htp->server == NULL) {
+        return -1;
+    }
 
 #ifndef EVHTP_DISABLE_SSL
     if (htp->ssl_ctx != NULL) {
@@ -3240,8 +3227,38 @@ evhtp_bind_sockaddr(evhtp_t * htp, struct sockaddr * sa, size_t sin_len, int bac
         }
     }
 #endif
-
     return 0;
+} /* evhtp_accept_socket */
+
+int
+evhtp_bind_sockaddr(evhtp_t * htp, struct sockaddr * sa, size_t sin_len, int backlog) {
+#ifndef WIN32
+    signal(SIGPIPE, SIG_IGN);
+#endif
+    evutil_socket_t fd;
+    int             on = 1;
+
+    fd = socket(sa->sa_family, SOCK_STREAM, 0);
+    evhtp_errno_assert(fd != -1);
+
+    evutil_make_socket_closeonexec(fd);
+    evutil_make_socket_nonblocking(fd);
+
+    setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, (void *)&on, sizeof(on));
+    setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (void *)&on, sizeof(on));
+
+    if (sa->sa_family == AF_INET6) {
+        int rc;
+
+        rc = setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on));
+        evhtp_errno_assert(rc != -1);
+    }
+
+    if (bind(fd, sa, sin_len) == -1) {
+        return -1;
+    }
+
+    return evhtp_accept_socket(htp, fd, backlog);
 } /* evhtp_bind_sockaddr */
 
 int
@@ -4378,4 +4395,5 @@ evhtp_make_request(evhtp_connection_t * c, evhtp_request_t * r,
 unsigned int
 evhtp_request_status(evhtp_request_t * r) {
     return htparser_get_status(r->conn->parser);
-}
+}
+