add_debugger_command(), but additionally takes parameters "usage"
and "flags".
* Added add_debugger_command_alias() which creates another name for an
existing command.
* Added print_debugger_command_usage() to print a command's usage.
* invoke_debugger_command() intercepts invocations with "--help" and
prints the command's usage text, if it is known. If unknown, the
command will be called normally.
* Made use of the new functions in debug.cpp.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23573 a95241bf-73f2-0310-859d-f6bbb57e9c96
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