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);