Make conftab.c independent of the current directory

read_builtin_tables() wanted to run in builtindir, and
read_custom_tables() wanted to run in configdir.  Bothersome.  Use new
fopenat() to relax those requirements.

The chdir() satisfying them are now superflous, remove them.

include/prototypes.h

@@ -323,6 +323,7 @@ extern int xundump(FILE *, char *, int *, int);
  */
 /* fnameat.c */
 extern char *fnameat(const char *, const char *);
+extern FILE *fopenat(const char *, const char *, const char *);
 /* fsize.c */
 extern int fsize(int);
 extern int blksize(int);

src/lib/common/conftab.c

@@ -42,6 +42,10 @@
 
 static int read_custom_table_file(char *);
 
+/*
+ * Read builtin configuration tables.
+ * Return 0 on success, -1 on failure.
+ */
 int
 read_builtin_tables(void)
 {
@@ -56,7 +60,7 @@ read_builtin_tables(void)
      */
     for (ep = empfile; ep->name; ep++) {
 	if (!EF_IS_GAME_STATE(ep->uid) && ep->file) {
-	    if ((fp = fopen(ep->file, "r")) == NULL) {
+	    if ((fp = fopenat(ep->file, "r", builtindir)) == NULL) {
 		fprintf(stderr, "Can't open %s for reading (%s)\n",
 			ep->file, strerror(errno));
 		return -1;
@@ -107,7 +111,7 @@ read_custom_table_file(char *fname)
     int lineno, res, n;
     FILE *fp;
 
-    if (!(fp = fopen(fname, "r"))) {
+    if (!(fp = fopenat(fname, "r", configdir))) {
 	fprintf(stderr, "Can't open config table %s for reading (%s)\n",
 		fname, strerror(errno));
 	return -1;

src/lib/gen/fnameat.c

@@ -71,3 +71,26 @@ fname_is_abs(const char *fname)
     return fname[0] == '/';
 #endif
 }
+
+/*
+ * Open a stream like fopen(), optionally relative to a directory.
+ * This is exactly like fopen(), except FNAME is interpreted relative
+ * to DIR if that is neither null nor empty.
+ */
+FILE *
+fopenat(const char *fname, const char *mode, const char *dir)
+{
+    char *fnat;
+    FILE *fp;
+
+    fnat = fnameat(fname, dir);
+    if (!fnat)
+	return NULL;
+
+    fp = fopen(fnat, mode);
+
+    if (fnat != fname)
+	free(fnat);
+
+    return fp;
+}

src/server/main.c

@@ -214,23 +214,8 @@ main(int argc, char **argv)
     if (emp_config(config_file) < 0)
 	exit(EXIT_FAILURE);
     ef_init();
-    if (chdir(configdir)) {
-	fprintf(stderr, "Can't chdir to %s (%s)\n",
-		configdir, strerror(errno));
-	exit(EXIT_FAILURE);
-    }
-    if (chdir(builtindir)) {
-	fprintf(stderr, "Can't chdir to %s (%s)\n",
-		builtindir, strerror(errno));
-	exit(EXIT_FAILURE);
-    }
     if (read_builtin_tables() < 0)
 	exit(EXIT_FAILURE);
-    if (chdir(configdir)) {
-	fprintf(stderr, "Can't chdir to %s (%s)\n",
-		configdir, strerror(errno));
-	exit(EXIT_FAILURE);
-    }
     if (read_custom_tables() < 0)
 	exit(EXIT_FAILURE);
     if (chdir(gamedir)) {