Avoid compiler warnings in comparing time_t.

Avoid compiler warnings in comparing time_t.

 src/dispnew.c (sit_for, Fsleep_for):
 src/keyboard.c (kbd_buffer_get_event):
 src/process.c (Faccept_process_output): Avoid compiler warnings when
 comparing a 32-bit time_t with a 64-bit INTMAX_MAX.