extopts: Fix for wrong initial return value of get_extopts.
extopts: Fix for initial emptying no-argument parameters.
main: Code cleaned from extopts debug output.

Printing usage is also refactored.
extopts: Fix for numerical types parsing.

Corrected cases when error state is returned.
extmods: Intoruced extmods extension.

Allows to operate utility modules.

Base operations:

1. find module by name with extmod_find();
2. execute module with extmod_exec().

To implement new module:

1. create module handler with prototype
  int handler(int argc, char *argv[]);
2. create optional extopts array for module if it suposed to have own
   parameters;
3. register module with macro EXTMOD_DECL:
  EXTMOD_DECL(name, handler, extopts)
  E.g:
  EXTMOD_DECL(tobin, do_tobin, &tobin_opts)
  Note: first argument (name of module) must be passed without quites.