file: Return NBD_FLAG_CAN_MULTI_CONN for the file plugin.

This allows multiple connections from a single client, and should be
safe assuming flush/FUA has been implemented correctly in the previous
commit.

Using the file plugin, but locating the file on /dev/shm, I obtained
the following results.

No multi-conn:

   read: IOPS=52.0k, BW=203MiB/s (213MB/s)(23.8GiB/120002msec)
  write: IOPS=52.0k, BW=203MiB/s (213MB/s)(23.8GiB/120002msec)

Multi-conn (-C 8):

   read: IOPS=122k, BW=477MiB/s (500MB/s)(55.9GiB/120002msec)
  write: IOPS=122k, BW=477MiB/s (500MB/s)(55.9GiB/120002msec)

plugins/file/file.c

@@ -250,6 +250,13 @@ file_get_size (void *handle)
   }
 }
 
+/* Allow multiple parallel connections from a single client. */
+static int
+file_can_multi_conn (void *handle)
+{
+  return 1;
+}
+
 static int
 file_can_trim (void *handle)
 {
@@ -507,6 +514,7 @@ static struct nbdkit_plugin plugin = {
   .open              = file_open,
   .close             = file_close,
   .get_size          = file_get_size,
+  .can_multi_conn    = file_can_multi_conn,
   .can_trim          = file_can_trim,
   .can_fua           = file_can_fua,
   .pread             = file_pread,