Add creation timestamp (birthtime) support for fuse.

I was keeping this #ifdef'd for reference with the MacFUSE change[1]
but on second thought, this is a FreeBSD-only header so the SVN
history should be enough.

Add missing padding while here.

Reference [1]:
http://code.google.com/p/macfuse/source/detail?spec=svn1686&r=1360