PJ: fix offset to advance for int types in var args.

Fixes crashes on 64-bit machines when substituting parameters into
dynamic strings, e.g. save flic menu.

Eventually this va_args string formatting business should be replaced
by something simpler and more portable.
PJ: fix casts.

In Celblit functions, the undo parameter is always undof.
PJ: fix save_redo_line parameter type.

xys should either be a Short_xy * or a Short_xy[2].
PJ: fix function pointer types.
PJ: fix missing return value.
PJ: fix missing field initializers.
PJ: fix out of range conditions.

ROOT_CHUNK_TYPE == (USHORT)-1.
PJ: fix & and && confusion.
PJ: clarify conditions that mix && and ||.
PJ: make assignments in conditionals more explicit.