Add 'summary' mode to Value.format_string

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.