io-util: make flush_fd() return how many bytes where flushed

This is useful so that callers know whether anything at all and how much
was flushed.

This patches through users of this functions to ensure that the return
values > 0 which may be returned now are not propagated in public APIs.

Also, users that ignore the return value are changed to do so explicitly
now.

src/basic/io-util.c

@@ -33,6 +33,7 @@ int flush_fd(int fd) {
                 .fd = fd,
                 .events = POLLIN,
         };
+        int count = 0;
 
         /* Read from the specified file descriptor, until POLLIN is not set anymore, throwing away everything
          * read. Note that some file descriptors (notable IP sockets) will trigger POLLIN even when no data can be read
@@ -52,7 +53,7 @@ int flush_fd(int fd) {
                         return -errno;
 
                 } else if (r == 0)
-                        return 0;
+                        return count;
 
                 l = read(fd, buf, sizeof(buf));
                 if (l < 0) {
@@ -61,11 +62,13 @@ int flush_fd(int fd) {
                                 continue;
 
                         if (errno == EAGAIN)
-                                return 0;
+                                return count;
 
                         return -errno;
                 } else if (l == 0)
-                        return 0;
+                        return count;
+
+                count += (int) l;
         }
 }