add --vgdb-prefix arg to callgrind_control
If valgrind is started with --vgdb-prefix arg, then callgrind_control
cannot find and control this valgrind.
So, add an (optional) argument to callgrind_control,
and have callgrind tool report the needed vgdb prefix argument
if the user supplied this arg.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13769 a5019735-40e9-0310-863c-91ae7b9d1cf9