Add gdb.Value.format_string ()

The str () function, called on a gdb.Value instance, produces a string
representation similar to what can be achieved with the print command,
but it doesn't allow to specify additional formatting settings, for
instance disabling pretty printers.

This patch introduces a new format_string () method to gdb.Value which
allows specifying more formatting options, thus giving access to more
features provided by the internal C function common_val_print ().

gdb/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	Add gdb.Value.format_string ().
	* python/py-value.c (copy_py_bool_obj):
	(valpy_format_string): Add gdb.Value.format_string ().
	* NEWS: Document the addition of gdb.Value.format_string ().

gdb/doc/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	* python.texi (Values From Inferior): Document
	gdb.Value.format_string ().

gdb/testsuite/ChangeLog:

2019-04-01  Marco Barisione  <mbarisione@undo.io>

	Test gdb.Value.format_string ().
	* gdb.python/py-format-string.exp: New test.
	* gdb.python/py-format-string.c: New file.
	* gdb.python/py-format-string.py: New file.

gdb/testsuite/gdb.python/py-format-string.py

@@ -0,0 +1,49 @@
+# Copyright (C) 2008-2019 Free Software Foundation, Inc.
+
+# 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/>.
+
+# This file is part of the GDB testsuite.  It tests python pretty
+# printers.
+
+import gdb
+
+class PointPrinter (object):
+    def __init__ (self, val):
+        self.val = val
+
+    def to_string (self):
+        return 'Pretty Point (%s, %s)' % (self.val['x'], self.val['y'])
+
+def test_lookup_function (val):
+    "Look-up and return a pretty-printer that can print val."
+
+    # Get the type.
+    type = val.type
+
+    # If it points to a reference, get the reference.
+    if type.code == gdb.TYPE_CODE_REF:
+        type = type.target ()
+
+    # Get the unqualified type, stripped of typedefs.
+    type = type.unqualified ().strip_typedefs ()
+
+    # Get the type name.
+    typename = type.tag
+
+    if typename == 'point':
+        return PointPrinter (val)
+
+    return None
+
+gdb.pretty_printers.append (test_lookup_function)