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.