Bug 1107968 - Add --debug-command to mach; r=ahal

People often seek to learn how mach commands work. A common way to do
this is to launch a debugger and step through the code as it is
executing. But this requires someone to first find and modify the mach
command. This involves overhead.

This patch adds a global --debug-command argument to mach. When present,
we launch an interactive debugger right before command dispatch. This
allows people to easily enter a debugger to see what mach commands are
doing, hopefully lowering the barrier to understanding and contributing.

--HG--
extra : rebase_source : 5ebfa7f5fd89f96dac5f7cb035ab520c63b32e55
extra : amend_source : c53c988763c328020b49aa5c6245df685b6f30bc