Reduce delay between backing up a file and saving new version

Reduce delay between backing up a file and saving new version

* lisp/files.el (basic-save-buffer-2):
Choose coding system for writing the file before backing it up.

* src/fileio.c (choose_write_coding_system): Make it callable from Lisp.
(Fwrite_region): If coding-system-for-write is set, don't call
choose_write_coding_system.
Move the last piece of choose_write_coding_system here.
(syms_of_fileio): Add choose-write-coding-system.

Fixes: debbugs:13522