Avoid format specifier %zu for Windows compatibility

Avoid format specifier %zu for Windows compatibility

- Replace %zu by %u and cast the size_t variable to (unsigned int). The
  cast should be safe as in all instances the number involved is small.

Note: mingw64 targets msvcrt.dll runtime that doesn't support %zu and
      print "zu" instead of the number. With -Wformat the compiler
      does warn that z is an unknown conversion type.

v2: Cast to (unsigned int) instead of (int).

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1470956309-31268-1-git-send-email-selva.nair@gmail.com>
URL: https://sourceforge.net/p/openvpn/mailman/message/35274787/
Signed-off-by: David Sommerseth <davids@openvpn.net>