Honor setgit bit on directories

Honor setgit bit on directories

Newly created files were always being created with the fsuid/fsgid
in the current users credentials.  This is correct except in the
case when the parent directory sets the 'setgit' bit.  In this
case according to posix the newly created file/directory should
inherit the gid of the parent directory.  Additionally, in the
case of a subdirectory it should also inherit the 'setgit' bit.

Finally, this commit performs a little cleanup of the vattr_t
initialization by moving it to a common helper function.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #262