filedesc: create a dedicated zone for struct filedesc0

Currently sizeof(struct filedesc0) is 1096 bytes, which means allocations from
malloc use 2048 bytes.

There is no easy way to shrink the structure <= 1024 an it is likely to grow in
the future.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274005 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f