build - Adjust _CMSG_ALIGN() for compatibility

build - Adjust _CMSG_ALIGN() for compatibility

* The argument to _CMSG_ALIGN() is commonly unsigned, fix compiler
  warnings by making the alignment constants unsigned.

* Also take a page from FreeBSD and cast the (n) argument to an
  unsigned quantity (size_t).

* Note that the alignment code is still hardwired to 4 bytes,
  and will eventually have to be changed to something more portable.
  For now do not change it as doing so would introduce an ABI
  incompatibility.

Rerported-by: rsmarples (Roy Marples)