Make econfig keys data and info work for relative names

File names in econfig need to be interpreted relative to configdir.
This wasn't the case everywhere for keys data and info.

Fix this by changing variables gamedir and infodir to hold absolute
names.  Change builtindir likewise, for consistency.  Store the values
from econfig in gamedir_conf, infodir_conf and builtindir_conf.

Uses new fnameat() to derive absolute names from possibly relative
ones.

include/prototypes.h

@@ -321,6 +321,8 @@ extern int xundump(FILE *, char *, int *, int);
 /*
  * src/lib/gen/ *.c 
  */
+/* fnameat.c */
+extern char *fnameat(const char *, const char *);
 /* fsize.c */
 extern int fsize(int);
 extern int blksize(int);