files: allow inserting also when started with the --noread option

The --noread option should apply only to the files listed on the
command line, but once nano has started, it should be possible to
insert other files into the current buffer or into a new buffer.

This fixes https://savannah.gnu.org/bugs/?61458.

Bug existed since version 2.3.3, since --noread was introduced.

src/nano.c

@@ -2476,6 +2476,9 @@ int main(int argc, char **argv)
 #endif
 	}
 
+	/* After handling the files on the command line, allow inserting files. */
+	UNSET(NOREAD_MODE);
+
 	/* If no filenames were given, or all of them were invalid things like
 	 * directories, then open a blank buffer and allow editing.  Otherwise,
 	 * switch from the last opened file to the next, that is: the first. */