Implement --merge-recursive-frames + provide VALGRIND_MONITOR_COMMAND client req.

In a big applications, some recursive algorithms have created
hundreds of thousands of stacktraces, taking a lot of memory.

Option --merge-recursive-frames=<number> tells Valgrind to
detect and merge (collapse) recursive calls when recording stack traces.
The value is changeable using the monitor command
'v.set merge-recursive-frames'.

Also, this provides a new client request: VALGRIND_MONITOR_COMMAND
allowing to execute a gdbsrv monitor command from the client
program.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13246 a5019735-40e9-0310-863c-91ae7b9d1cf9