use int instead of bool for C function rt_unloadLibrary

- This should actually be ABI compatible but it's cleaner
  for documentation purpose.
Merge pull request #623 from dawgfoto/rt_loadLibrary

use int instead of bool for C function rt_unloadLibrary
Add declaration of CP_UTF8
Merge pull request #637 from JakobOvrum/cputf8

Add declaration of CP_UTF8