Add command execution runtime (lcmd.c, lcmd.h)

Implements luaB_command: fork/execvp with pipe-captured stdout/stderr,
argv parsing with quoting/escaping, and non-blocking pipe reading via
select(). Backs the __command() calls emitted by the backtick parser.

Resolves #03 and #04.

lcmd.h

@@ -0,0 +1,14 @@
+/*
+** $Id: lcmd.h $
+** Command execution for backtick syntax
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lcmd_h
+#define lcmd_h
+
+#include "lua.h"
+
+int luaB_command (lua_State *L);
+
+#endif