Save breakpoints so they are automatically pending

PR breakpoints/18183 points out that breakpoints saved with the "save
breakpoints" command can be slightly inconvenient, depending on "set
breakpoint pending".

This patch makes use of the new "with" command to save breakpoints
such that they will automatically be made pending, if necessary, when
being restored.

Unfortunately, reloading a breakpoint saved this way will also print:

    No symbol table is loaded.  Use the "file" command.

This seems strange to me, and also a bit useless, but changing
create_breakpoint not to call exception_print in the AUTO_BOOLEAN_TRUE
case caused other regressions, so I've left it as-is for the time
being.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18183
Reviewed-By: Keith Seitz <keiths@redhat.com>