Fix sig.c build warning
Fix sig.c build warning

Refactor new kill/shutdown implementation from 3e0063e to fix the
regression in compiler output:

sig.c: In function ‘kill_callback’:
sig.c:222:12: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
  222 |  kill(pid, (int)context);
      |            ^

Also, some minor renames and simplificactions to match project style.

Signed-off-by: Joachim Wiberg <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/finit,troglobit/finit