Add support for extattr_* family of functions

[notgull]

The equivalent of the Xattr system in BSD is extattr_*. It would be nice to expose this.

Notable functions:

• extattr_delete_fd
• extattr_delete_link
• extattr_get_fd
• extattr_get_link
• extattr_list_fd
• extattr_list_link
• extattr_set_fd
• extattr_set_link,

cc Stebalien/xattr#41