add COMMAND_HANDLER and COMMAND_HELPER macros

add COMMAND_HANDLER and COMMAND_HELPER macros

The COMMAND_HANDLER and COMMAND_HELPER macros allow commands to be
defined in a manner that decouples them from the exact order and type of
their parameters.  Once converted, incremental changes to the command
handler type can be addressed in incremental patches that do not need to
touch the entire tree.

These macros' implementation, __COMMAND_HANDLER, is used to define the
new command_handler_t type, and additional patches will use it to derive
new macros to define extended command types (e.g. flash, nand, pld).
The CALL_COMMAND_HANDLER provides a means of calling helpers or nested
handlers from withing a command handler.

This patch uses C99 varadic macro expansion.  Please report compilers
that cannot handle this code.