Fix bug when time_t is unsigned and as wide as intmax_t.

Fix bug when time_t is unsigned and as wide as intmax_t.

* lisp.h (WAIT_READING_MAX): New macro.
* dispnew.c (Fsleep_for, sit_for):
* keyboard.c (kbd_buffer_get_event):
* process.c (Faccept_process_output):
Use it to avoid bogus compiler warnings with obsolescent GCC versions.
This improves on the previous patch, which introduced a bug
when time_t is unsigned and as wide as intmax_t.
See <http://bugs.gnu.org/9000#51>.