shutdown: when dying, do not install/restore a handler for Ctrl+C

First, we don't want the writing of an emergency file to be interrupted
by the user.  But more important: the routine for restoring the handler
also disables SIGINT, which would leave the terminal with a non-working
Ctrl+C.

Saving an emergency file calls write_file() in a unique manner: with
thefile == NULL, fullbuffer == FALSE (even though the entire buffer
will be saved, of course) and tmp == TRUE (even though it is not a
temporary file, as it will persist after nano exits).  But in fact
we want the handler for Ctrl+C installed only for normal files, not
for temporary files and not for emergency files -- the user should
not be able to interrupt the writing of those.

This fixes https://savannah.gnu.org/bugs/?61237.

Bug existed since version 4.3, commit 8550c6bd.

src/files.c

@@ -1848,7 +1848,8 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 
 #ifndef NANO_TINY
 		block_sigwinch(TRUE);
-		install_handler_for_Ctrl_C();
+		if (!tmp)
+			install_handler_for_Ctrl_C();
 #endif
 
 		/* Now open the file.  Use O_EXCL for an emergency file. */
@@ -1856,7 +1857,8 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 					O_APPEND : (tmp ? O_EXCL : O_TRUNC)), permissions);
 
 #ifndef NANO_TINY
-		restore_handler_for_Ctrl_C();
+		if (!tmp)
+			restore_handler_for_Ctrl_C();
 		block_sigwinch(FALSE);
 #endif