[RFC] add 'save-breakpoints' command

[Klee Dienes]

The way we addressed this in our sources was to add a new
'future-break' command, and modify the way we handle breakpoints that
cannot be inserted.

When our GDB is unable to set a breakpoint that has previously been
set successfuly, it marks the breakpoint as 'shlib_disabled'.  Then
when shared libraries are loaded, GDB goes through the list of
shlib_disabled breakpoints and tries to re-set them.

The 'future-break' command allows one to specify a breakpoint that
starts off as 'shlib_disabled' instead of generating an error if it
can't be set immediately.

I'm not entirely happy with the future-break command, and particularly
with its interaction with the save-breakpoints mechanism (if you set a
breakpoint that is in a shared library after shared libraries have
been loaded, you have to remember to use the future-break command, not
the break command).  This isn't a particularly big problem for us,
since we do decent good job of pre-loading symbols for all our shared
libraries --- but it's enough that I thought it better to treat as a
separate issue from the save-breakpoints command.