Revert 2013-01-31 change that decides coding system before backing up

Revert 2013-01-31 change that decides coding system before backing up

It causes a more serious problem than the one it solves.
This closes bug#18141, and reopens bug#13522.

* lisp/files.el (basic-save-buffer-2): Revert 2013-01-31 change.

* src/fileio.c: Revert 2013-01-31 change.
(choose_write_coding_system): No longer callable from Lisp.
Move last piece back here from Fwrite_region.
(Fwrite_region, syms_of_fileio): Update for above changes.

* test/automated/data/files-bug18141.el.gz: New file.
* test/automated/files.el (files-test-bug-18141-file):
New variable and test.