bug#3016: 23.0.92.2; offer to make a directory when saving if it is needed

[Daniel Clemente]

1. Do: C-x C-f /tmp/doesnotexist/myfile RET. Emacs tells you about 
make-directory
2. Type something.
3. C-x C-s. Error: no such directory

I suggest that Emacs not only tell the user about make-directory, but actually 
ask him/her in a y-n question if the directory should be created. This should 
happen on save.

The script could be then:

a. User does: C-x C-f /tmp/doesnotexist/myfile RET
b. Emacs warns: Warning: Directory /tmp/doesnotexist does not exist
c. User types something
d. User does: C-x C-s
e. Emacs asks: Create directory /tmp/doesnotexist? (y/n)
f1. If yes, it (and the above directories) are created
f2. If not, issue an error. The user can then do M-x make-directory as needed


This allows a faster save with just one extra key (C-x C-s y) instead of having 
to type M-x make-directory /tmp/doesnotexist RET C-x C-s