Implement startup configuration file support (issue #08)

Load ~/.config/lush/config and config.d/*.lua at REPL startup,
respecting XDG_CONFIG_HOME. Suppressed by -E flag.

lua.c

@@ -13,6 +13,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include <dirent.h>
+#include <limits.h>
 #include <signal.h>
 
 #include "lua.h"
@@ -722,6 +724,79 @@ static void doREPL (lua_State *L) {
 #endif
 
 
+/*
+** {==================================================================
+** Configuration file support
+** ===================================================================
+*/
+
+static const char *get_config_dir (char *buf, size_t bufsize) {
+  const char *xdg = getenv("XDG_CONFIG_HOME");
+  const char *home = getenv("HOME");
+  if (xdg && xdg[0] != '\0')
+    snprintf(buf, bufsize, "%s/lush", xdg);
+  else if (home && home[0] != '\0')
+    snprintf(buf, bufsize, "%s/.config/lush", home);
+  else
+    return NULL;
+  return buf;
+}
+
+
+static int cmpstr (const void *a, const void *b) {
+  return strcmp(*(const char **)a, *(const char **)b);
+}
+
+
+static void run_config_dir (lua_State *L, const char *dirpath) {
+  char *names[256];
+  int count = 0;
+  int i;
+  struct dirent *entry;
+  DIR *d = opendir(dirpath);
+  if (d == NULL) return;  /* directory doesn't exist — skip silently */
+
+  /* collect .lua filenames */
+  while ((entry = readdir(d)) != NULL && count < 256) {
+    size_t len = strlen(entry->d_name);
+    if (len > 4 && strcmp(entry->d_name + len - 4, ".lua") == 0)
+      names[count++] = strdup(entry->d_name);
+  }
+  closedir(d);
+
+  /* sort lexicographically */
+  qsort(names, (size_t)count, sizeof(char *), cmpstr);
+
+  /* execute each file */
+  for (i = 0; i < count; i++) {
+    char path[PATH_MAX];
+    snprintf(path, sizeof(path), "%s/%s", dirpath, names[i]);
+    dofile(L, path);  /* errors reported but not fatal */
+    free(names[i]);
+  }
+}
+
+
+static void handle_lushconfig (lua_State *L) {
+  char configdir[PATH_MAX];
+  char path[PATH_MAX];
+
+  if (get_config_dir(configdir, sizeof(configdir)) == NULL)
+    return;  /* no HOME or XDG_CONFIG_HOME — skip silently */
+
+  /* 1. Run ~/.config/lush/config if it exists */
+  snprintf(path, sizeof(path), "%s/config", configdir);
+  if (access(path, R_OK) == 0)
+    dofile(L, path);  /* errors are reported but not fatal */
+
+  /* 2. Run config.d/ lua files in sorted order */
+  snprintf(path, sizeof(path), "%s/config.d", configdir);
+  run_config_dir(L, path);
+}
+
+/* }================================================================== */
+
+
 /*
 ** Main body of stand-alone interpreter (to be called in protected mode).
 ** Reads the options and handles them all.
@@ -757,11 +832,16 @@ static int pmain (lua_State *L) {
     if (handle_script(L, argv + script) != LUA_OK)
       return 0;  /* interrupt in case of error */
   }
-  if (args & has_i)  /* -i option? */
+  if (args & has_i) {  /* -i option? */
+    if (!(args & has_E))
+      handle_lushconfig(L);
     doREPL(L);  /* do read-eval-print loop */
+  }
   else if (script < 1 && !(args & (has_e | has_v))) { /* no active option? */
     if (lua_stdin_is_tty()) {  /* running in interactive mode? */
       print_version();
+      if (!(args & has_E))
+        handle_lushconfig(L);
       doREPL(L);  /* do read-eval-print loop */
     }
     else dofile(L, NULL);  /* executes stdin as a file */