* src/ttt-board.h:
* src/ttt-board.c: (ttt_board_init), (ttt_board_init_from_string),
(ttt_board_to_string), (ttt_board_write): Add some stub functions
for Richard and Kevin to have some practice
implementing. Functions to represent a board and to go to/from a
string.
* src/x.h:
* src/x.c: (xasprintf), (xvasprintf), (xpipe), (xfork), (xmalloc),
(xcalloc), (xrealloc), (xfdopen), (xstrdup), (xfwrite): Several
utility functions that make system calls, check the result, and
exit on any error. This will simplify the error checking needed in
programs using these functions.