bug#25303: 24.5; `write-region' enhancement request for MUSTBENEW parameter

[Drew Adams <drew.adams@oracle.com>]

If parameter MUSTBENEW is not `excl' and is non-nil then the user is
prompted to confirm overwriting.  The enhancement would be that if the
value is a string then use that as the confirmation prompt.

The would allow for more specific prompting, such as saying something
more than that the file exists - something that can help the user decide
whether to overwrite.

This is pretty much backward compatible: In existing code it is likely
that few, if any, non-nil and non-`excl' arg values are strings.  And in
the case of any that are, the use of the string as a prompt will make
things obvious to a maintainer, if not immediately to the user.

(This is certainly a lot more backward-compatible than was the change
Emacs made to respect the particular value `excl' (in Emacs 21 or 22).
That broke all existing code that was written to the previous spec that
the non-nil arg meant prompt to CONFIRM, not MUSTBENEW.)


In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'