add struct command_invocation for COMMAND_HANDLER

add struct command_invocation for COMMAND_HANDLER

Adds the command_invocation structure to encapsulate parameters for
all COMMAND_HANDLER routines.  Rather than passing several arguments
to each successive subroutine, a single pointer may be passed around.

Changes the CMD_* macros to reference the new fields.

Updates run_command to create an instance and pass it to the handler.