* buffer.h (struct buffer): Use time_t, not int, for a time stamp.

* buffer.h (struct buffer): Use time_t, not int, for a time stamp.

This fixes a Y2038 bug on 64-bit hosts.
* buffer.c (reset_buffer):
* fileio.c (Fdo_auto_save, Fset_buffer_auto_saved)
(Fclear_buffer_auto_save_failure):
Use 0, not -1, to represent an unset failure time, since time_t
might not be signed.