Implement shell builtins: cd, exec, umask (issue #15)

Add builtin dispatch in lcmd.c that checks __builtins table before
fork(), so cd/exec/umask operate on the shell process itself.

lcmd.c

@@ -20,6 +20,7 @@
 #include "lua.h"
 #include "lauxlib.h"
 #include "lcmd.h"
+#include "lbuiltin.h"
 
 
 /* ===== argv parser ===== */
@@ -555,6 +556,30 @@ static int exec_pipeline (lua_State *L, char **stages, int nstages,
 }
 
 
+/* ===== builtin dispatch ===== */
+
+/*
+** Check if argv[0] is a shell builtin. If so, call it and push the
+** result table. Returns 1 if handled, 0 if not a builtin.
+*/
+static int try_builtin (lua_State *L, ParsedArgs *pa) {
+  int i;
+  if (lua_getglobal(L, "__builtins") != LUA_TTABLE) {
+    lua_pop(L, 1);
+    return 0;
+  }
+  if (lua_getfield(L, -1, pa->argv[0]) != LUA_TFUNCTION) {
+    lua_pop(L, 2);
+    return 0;
+  }
+  lua_remove(L, -2);  /* remove __builtins, keep function */
+  for (i = 0; i < pa->argc; i++)
+    lua_pushstring(L, pa->argv[i]);
+  lua_call(L, pa->argc, 1);
+  return 1;
+}
+
+
 /* ===== luaB_command ===== */
 
 int luaB_command (lua_State *L) {
@@ -599,6 +624,12 @@ int luaB_command (lua_State *L) {
     return 1;
   }
 
+  /* check for shell builtin */
+  if (try_builtin(L, &pa)) {
+    free_argv(&pa);
+    return 1;
+  }
+
   /* create pipes */
   if (pipe(out_pipe) != 0) {
     free_argv(&pa);
@@ -731,6 +762,13 @@ int luaB_interactive (lua_State *L) {
     return 0;
   }
 
+  /* check for shell builtin */
+  if (try_builtin(L, &pa)) {
+    free_argv(&pa);
+    lua_setglobal(L, "_");
+    return 0;
+  }
+
   /* ignore SIGINT, SIGQUIT, SIGPIPE in parent */
   memset(&sa_new, 0, sizeof(sa_new));
   sa_new.sa_handler = SIG_IGN;