basic: move acquire_data_fd() and fd_duplicate_data_fd() to new data-fd-util.c

fd_duplicate_data_fd() is renamed to copy_data_fd(). This makes
the two functions have nicely similar names.

Now fd-util.[ch] is again about low-level file descriptor manipulations.
copy_data_fd() is a complex function that internally wraps the other
functions in copy.c. I want to move copy.c and the whole cluster of
related code from basic/ to shared/ later on, and this is a preparatory
step for that.

src/basic/fd-util.h

@@ -76,10 +76,6 @@ enum {
         ACQUIRE_NO_REGULAR  = 1 << 4,
 };
 
-int acquire_data_fd(const void *data, size_t size, unsigned flags);
-
-int fd_duplicate_data_fd(int fd);
-
 int fd_move_above_stdio(int fd);
 
 int rearrange_stdio(int original_input_fd, int original_output_fd, int original_error_fd);