Hide shell functions from _G using upvalues on the main chunk

Store __command, __interactive, __getenv, __setenv as upvalues
populated by lua_load() from the registry, keeping them invisible
to user code while accessible to the parser's codegen.

lcmd.h

@@ -9,7 +9,17 @@
 
 #include "lua.h"
 
-int luaB_command (lua_State *L);
-int luaB_interactive (lua_State *L);
+/* Upvalue indices for shell functions on the main chunk.
+** Index 0 is _ENV (standard). Indices 1-4 are lush shell functions. */
+#define LUSH_UPVAL_COMMAND      1
+#define LUSH_UPVAL_INTERACTIVE  2
+#define LUSH_UPVAL_GETENV       3
+#define LUSH_UPVAL_SETENV       4
+#define LUSH_NUM_UPVALS         5  /* total: _ENV(0) + 4 shell functions */
+
+int lushCmd_command (lua_State *L);
+int lushCmd_interactive (lua_State *L);
+int lushCmd_getenv (lua_State *L);
+int lushCmd_setenv (lua_State *L);
 
 #endif