Here's a first cut at the "bookmark" functionality discussed earlier.

This should be target-agnostic -- I've added two target methods,
to_get_bookmark and to_goto_bookmark.  For remote targets, we use
q/Q query syntax.

Seems to work well in process record.

Summary:
   bookmark (no argument)
     Save a reference to the current position in the replay log.
     Doesn't care whether replay log is precord or arbitrary remote
     target.
   delete bookmark <N>
     Delete the bookmark numbered N (just like breakpoint numbers)
   goto-bookmark <N>
     Return the replay log to the state of the bookmark numbered N.
   info bookmarks
     List bookmarks.


Bookmarks are saved internally in the form of a string, which is
provided by and returned to the target.

There is a back-door -- if the user understands the format of
the string that is used by the target to represent bookmarks,
he can supply a quoted string (single or double quotes) eg.:

   (gdb) bookmark '12345'

which will be passed along to the target, even if no corresponding
bookmark exists in gdb's internal list.