Before writing a file out, nano should check that the file on disk
hasn't been modified since it was read -- not only for the normal
"Write Out" action (^O), but also for "Save File" (future ^S) and
for "Save and Exit" (^X when --tempfile is used).

When writing fails and --tempfile is in effect, don't go on to prompt
for a file name; instead let the user decide what she wants to do.

This fixes https://savannah.gnu.org/bugs/?51040

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Signed-off-by: Viorel Bota <botaviorel@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>