bug#992: coding system for next command not honoured with C-u 0 C-x s

[Francesco Potorti`]

To reproduce:

emacs -Q
C-x C-f aaa RET                         -- create a file aa
<write anything, with carriage return>  -- write random text
C-x RET c dos RET C-x C-s               -- save it in Dos mode
C-x k RET                               -- kill the buffer
C-x C-f aaa RET                         -- reload the file
<write anything>                        -- modify it
C-x RET c unix RET M-0 C-x C-s          -- save in Unix mode, no backup

At this point, the buffer should be in Unix mode, but instead it is in
Dos mode.  Accordingly, the file is saved in Dos mode.  The bug only
happens with M-0 C-x C-s, not with plain C-x C-s.