Initialize pad memory to pacify stack protectors

Initialize pad memory to pacify stack protectors

- Initialize memory area that is being encrypted, because it is allocated
  rounded up to CBLKSIZE, and the last bytes are not used. If stack
  protector is active, it detects access to uninitialized memory.
- Change default pamdir to /lib/${host_cpu}-${host_os}/security.
- Fix a couple of compilation warnings.