Make ptrace-based launchers able to handle --help, --version etc.

Make ptrace-based launchers able to handle --help, --version etc.
Problem is that --help etc are handled by the tool exe.  But a
ptrace-based launch scheme can't run "no program" if the user just
types "valgrind --help" because the launcher depends on starting the
client first and only then attaching valgrind to it using ptrace.  So
instead provide a dummy do-nothing program to run when no program is
specified.  m_main notices this and acts as if there really had been
no program specified.

This has no effect at all on Linux/ELF program launching.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6653