unix: Implement -m option (execute module from stdlib).

Support for packages as argument not implemented, but otherwise error and
exit handling should be correct. This for example will allow to do:

pip-micropython install micropython-test.pystone
micropython -m test.pystone
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 80c4c77..af93aac 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -286,6 +286,14 @@
 
                 module_obj = mp_obj_new_module(mod_name);
 
+                // if args[3] (fromtuple) has magic value False, set up
+                // this module for command-line "-m" option (set module's
+                // name to __main__ instead of real name).
+                if (i == mod_len && fromtuple == mp_const_false) {
+                    mp_obj_module_t *o = module_obj;
+                    mp_obj_dict_store(o->globals, MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR___main__));
+                }
+
                 if (stat == MP_IMPORT_STAT_DIR) {
                     DEBUG_printf("%s is dir\n", vstr_str(&path));
                     // https://docs.python.org/3/reference/import.html