Add 'summary' mode to Value.format_string

This adds a 'summary' mode to Value.format_string and to
gdb.print_options.  For the former, it lets Python code format values
using this mode.  For the latter, it lets a printer potentially detect
if it is being called in a backtrace with 'set print frame-arguments'
set to 'scalars'.

I considered adding a new mode here to let a pretty-printer see
whether it was being called in a 'backtrace' context at all, but I'm
not sure if this is really desirable.

gdb/python/py-prettyprint.c

@@ -753,6 +753,8 @@ gdbpy_print_options (PyObject *unused1, PyObject *unused2)
 		      opts.static_field_print) < 0
       || set_boolean (result.get (), "deref_refs",
 		      opts.deref_ref) < 0
+      || set_boolean (result.get (), "summary",
+		      opts.summary) < 0
       || set_unsigned (result.get (), "max_elements",
 		       opts.print_max) < 0
       || set_unsigned (result.get (), "max_depth",