Fix GCC -Wsign-compare errors in the converted C modules

GCC enables -Wsign-compare with -Wall for C++ (clang does not), which
turned a few signed/unsigned comparisons in base64, netutils, server and
manager into errors; udp.cpp needed <ctime>.

src/base64.cpp

@@ -91,7 +91,7 @@ char *base64_encode(char *out, int out_size, const uint8_t *in, int in_size)
     int i_shift = 0;
     int bytes_remaining = in_size;
 
-    if (in_size >= UINT_MAX / 4 ||
+    if ((unsigned)in_size >= UINT_MAX / 4 ||
         out_size < BASE64_SIZE(in_size))
         return NULL;
     ret = dst = out;

src/manager.cpp

@@ -605,7 +605,7 @@ get_server(char *buf, int len)
     int valid = 1;
     if (obj->type == json_object) {
         int i = 0;
-        for (i = 0; i < obj->u.object.length; i++) {
+        for (i = 0; (unsigned int)i < obj->u.object.length; i++) {
             char *name        = obj->u.object.values[i].name;
             json_value *value = obj->u.object.values[i].value;
             if (strcmp(name, "server_port") == 0) {
@@ -699,7 +699,7 @@ parse_traffic(char *buf, int len, char *port, uint64_t *traffic)
 
     if (obj->type == json_object) {
         int i = 0;
-        for (i = 0; i < obj->u.object.length; i++) {
+        for (i = 0; (unsigned int)i < obj->u.object.length; i++) {
             char *name        = obj->u.object.values[i].name;
             json_value *value = obj->u.object.values[i].value;
             if (value->type == json_integer) {
@@ -1032,7 +1032,7 @@ manager_recv_cb(SS_P_ ss_io *w, int revents)
             }
             if (pos >= BUF_SIZE - entry_pos) {
                 if (sendto(manager->fd, buf, pos, 0, (struct sockaddr *)&claddr, len)
-                    != pos) {
+                    != (ssize_t)pos) {
                     ERROR("list_sendto");
                 }
                 memset(buf, 0, BUF_SIZE);
@@ -1045,7 +1045,7 @@ manager_recv_cb(SS_P_ ss_io *w, int revents)
         strcpy(buf + (pos > 2 ? pos - 1 : 1), "\n]"); // Remove trailing ","
         pos = strlen(buf);
         if (sendto(manager->fd, buf, pos, 0, (struct sockaddr *)&claddr, len)
-            != pos) {
+            != (ssize_t)pos) {
             ERROR("list_sendto");
         }
     } else if (strcmp(action, "remove") == 0) {
@@ -1107,7 +1107,7 @@ manager_recv_cb(SS_P_ ss_io *w, int revents)
                     goto ERROR_MSG;
                 }
                 if (sendto(manager->fd, buf, pos, 0, (struct sockaddr *)&claddr, len)
-                    != pos) {
+                    != (ssize_t)pos) {
                     ERROR("ping_sendto");
                 }
                 memset(buf, 0, BUF_SIZE);
@@ -1132,7 +1132,7 @@ manager_recv_cb(SS_P_ ss_io *w, int revents)
         }
 
         if (sendto(manager->fd, buf, pos, 0, (struct sockaddr *)&claddr, len)
-            != pos) {
+            != (ssize_t)pos) {
             ERROR("ping_sendto");
         }
     }

src/netutils.cpp

@@ -341,7 +341,7 @@ validate_hostname(const char *hostname, const int hostname_len)
 int
 is_ipv6only(ss_addr_t *servers, size_t server_num, int ipv6first)
 {
-    int i;
+    size_t i;
     for (i = 0; i < server_num; i++) {
         struct sockaddr_storage storage;
         memset(&storage, 0, sizeof(struct sockaddr_storage));

src/server.cpp

@@ -226,7 +226,7 @@ stat_update_cb(SS_P_ ss_timer *watcher, int revents)
         strncpy(svaddr.sun_path, manager_addr, sizeof(svaddr.sun_path) - 1);
 
         if (sendto(sfd, resp, strlen(resp) + 1, 0, (struct sockaddr *)&svaddr,
-                   sizeof(struct sockaddr_un)) != msgLen) {
+                   sizeof(struct sockaddr_un)) != (ssize_t)msgLen) {
             ERROR("stat_sendto");
             ss_socket_close(sfd);
             return;