Addition of v.info scheduler monitor command

Addition of v.info scheduler monitor command

Monitor command useful for debugging/investigation of Valgrind unwinder and/or
gdbserver/gdb stack trace.

The Valgrind unwinder has some limitations compared to the GDB unwinder.
(see e.g. 278972).

With this monitor command, it is easy to see if the Valgrind unwinder
produces something different than the GDB unwinder.

Fixes #279212.  (Philippe Waroquiers, philippe.waroquiers@skynet.be)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12203