Expose lush global as a standard library (issue #26)

Register luaopen_lush() in stdlibs so the lush table is available as a
global. Users can now extend the shell by adding functions to
lush.builtins. The OP_LUSH VM access via integer keys is preserved.

lcmd.c

@@ -1199,3 +1199,34 @@ int lushCmd_setenv (lua_State *L) {
   }
   return 0;
 }
+
+
+/* ===== lush standard library ===== */
+
+static const luaL_Reg lushlib[] = {
+  {"command", lushCmd_command},
+  {"interactive", lushCmd_interactive},
+  {"getenv", lushCmd_getenv},
+  {"setenv", lushCmd_setenv},
+  {"subcmd", lushCmd_subcmd},
+  {NULL, NULL}
+};
+
+LUAMOD_API int luaopen_lush (lua_State *L) {
+  luaL_newlib(L, lushlib);
+  /* also store integer-keyed entries for OP_LUSH VM access */
+  lua_pushcfunction(L, lushCmd_command);
+  lua_rawseti(L, -2, LUSH_OP_COMMAND + 1);
+  lua_pushcfunction(L, lushCmd_interactive);
+  lua_rawseti(L, -2, LUSH_OP_INTERACTIVE + 1);
+  lua_pushcfunction(L, lushCmd_getenv);
+  lua_rawseti(L, -2, LUSH_OP_GETENV + 1);
+  lua_pushcfunction(L, lushCmd_setenv);
+  lua_rawseti(L, -2, LUSH_OP_SETENV + 1);
+  lua_pushcfunction(L, lushCmd_subcmd);
+  lua_rawseti(L, -2, LUSH_OP_SUBCMD + 1);
+  /* store in registry for OP_LUSH access */
+  lua_pushvalue(L, -1);
+  lua_rawseti(L, LUA_REGISTRYINDEX, LUA_RIDX_LUSH);
+  return 1;
+}