handle signal interruption

[meow-watermelon]

the memory cannot be cleaned up properly if users send interruptable signals like SIGINT:

^C==58715== 
==58715== Process terminating with default action of signal 2 (SIGINT)
==58715==    at 0x4950127: clock_nanosleep@@GLIBC_2.17 (clock_nanosleep.c:71)
==58715==    by 0x4962C96: nanosleep (nanosleep.c:25)
==58715==    by 0x49745D2: sleep (sleep.c:55)
==58715==    by 0x401A14: main (memdoor.c:227)
==58715== 
==58715== HEAP SUMMARY:
==58715==     in use at exit: 3,354 bytes in 8 blocks
==58715==   total heap usage: 54 allocs, 46 frees, 60,258 bytes allocated
==58715== 
==58715== LEAK SUMMARY:
==58715==    definitely lost: 0 bytes in 0 blocks
==58715==    indirectly lost: 0 bytes in 0 blocks
==58715==      possibly lost: 0 bytes in 0 blocks
==58715==    still reachable: 3,354 bytes in 8 blocks
==58715==         suppressed: 0 bytes in 0 blocks
==58715== Reachable blocks (those to which a pointer was found) are not shown.
==58715== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==58715== 
==58715== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

[meow-watermelon]

handling non-global pointers is tricky and complex. i will go ahead and close this issue.