Implement putstr and putstrn in ui_file

In my tour of the ui_file subsystem, I found that fputstr and fputstrn
can be simplified.  The _filtered forms are never used (and IMO
unlikely to ever be used) and so can be removed.  And, the interface
can be simplified by removing a callback function and moving the
implementation directly to ui_file.

A new self-test is included.  Previously, I think nothing was testing
this code.

Regression tested on x86-64 Fedora 34.

gdb/unittests/ui-file-selftests.c

@@ -0,0 +1,62 @@
+/* Self tests for ui_file
+
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "gdbsupport/selftest.h"
+#include "ui-file.h"
+
+namespace selftests {
+namespace file {
+
+static void
+check_one (const char *str, int quoter, const char *result)
+{
+  string_file out;
+  out.putstr (str, quoter);
+  SELF_CHECK (out.string () == result);
+}
+
+static void
+run_tests ()
+{
+  check_one ("basic stuff: \\", '\\',
+	     "basic stuff: \\\\");
+  check_one ("more basic stuff: \\Q", 'Q',
+	     "more basic stuff: \\\\\\Q");
+  check_one ("more basic stuff: \\Q", '\0',
+	     "more basic stuff: \\Q");
+
+  check_one ("weird stuff: \x1f\x90\n\b\t\f\r\033\007", '\\',
+	     "weird stuff: \\037\\220\\n\\b\\t\\f\\r\\e\\a");
+
+  scoped_restore save_7 = make_scoped_restore (&sevenbit_strings, true);
+  check_one ("more weird stuff: \xa5", '\\',
+	     "more weird stuff: \\245");
+}
+
+} /* namespace file*/
+} /* namespace selftests */
+
+void _initialize_ui_file_selftest ();
+void
+_initialize_ui_file_selftest ()
+{
+  selftests::register_test ("ui-file",
+			    selftests::file::run_tests);
+}