Run-time version information is now available

Run-time version information is now available

Run-time version information is available as exported four integers:
- const int      nl_ver_num = LIBNL_VER_NUM;
- const int      nl_ver_maj = LIBNL_VER_MAJ;
- const int      nl_ver_min = LIBNL_VER_MIN;
- const int      nl_ver_mic = LIBNL_VER_MIC;

The purpose of this is to get version of compiled library as run time.
Use cases:
- To know exact version of the library in Python's ctypes module,
  Say, to find out if nl_cache_mngr_alloc() allow sk=NULL

- To make sure that the version of the loaded library corresponds to the
  version of headers (for the paranoid). Say, to check:

  if (LIBNL_VER_NUM != nl_ver_num)
      exit(1);