files: don't change embedded newlines into nulls in filenames

Because changing anything to a null effectively means to truncate
the name.

This fixes https://savannah.gnu.org/bugs/?49868
and fixes https://savannah.gnu.org/bugs/?49874.

src/browser.c

@@ -237,9 +237,6 @@ char *do_browser(char *path)
 		continue;
 	    }
 
-	    /* Convert newlines to nulls in the directory name. */
-	    sunder(answer);
-
 	    path = free_and_assign(path, real_dir_from_tilde(answer));
 
 	    /* If the given path is relative, join it with the current path. */

src/files.c

@@ -1173,9 +1173,6 @@ void do_insertfile(void)
 		current_was_at_top = (openfile->edittop == openfile->fileage);
 	    }
 
-	    /* Convert newlines to nulls in the given filename. */
-	    sunder(answer);
-
 #ifndef NANO_TINY
 	    if (execute) {
 #ifndef DISABLE_MULTIBUFFER
@@ -2320,15 +2317,10 @@ int do_writeout(bool exiting)
 #endif
 
 	    if (method == OVERWRITE) {
-		size_t answer_len = strlen(answer);
 		bool name_exists, do_warning;
 		char *full_answer, *full_filename;
 		struct stat st;
 
-		/* Convert newlines to nulls, just before we get the
-		 * full path. */
-		sunder(answer);
-
 		full_answer = get_full_path(answer);
 		full_filename = get_full_path(openfile->filename);
 		name_exists = (stat((full_answer == NULL) ?
@@ -2340,10 +2332,6 @@ int do_writeout(bool exiting)
 				answer : full_answer, (full_filename == NULL) ?
 				openfile->filename : full_filename) != 0);
 
-		/* Convert nulls to newlines.  answer_len is the
-		 * string's real length. */
-		unsunder(answer, answer_len);
-
 		free(full_filename);
 		free(full_answer);
 
@@ -2399,9 +2387,6 @@ int do_writeout(bool exiting)
 #endif
 	    }
 
-	    /* Convert newlines to nulls, just before we save the file. */
-	    sunder(answer);
-
 	    /* Here's where we allow the selected text to be written to
 	     * a separate file.  If we're using restricted mode, this
 	     * function is disabled, since it allows reading from or