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/core/execute.c

@@ -46,6 +46,7 @@
 #include "cgroup-setup.h"
 #include "chown-recursive.h"
 #include "cpu-set-util.h"
+#include "data-fd-util.h"
 #include "def.h"
 #include "env-file.h"
 #include "env-util.h"