Support for Linux extended attributes.

Support for Linux extended attributes.

This commit adds six calls to support Linux extended attributes.
They are:
  getxattrs     list all extended attributes for a file or directory
  setxattr      add/replace an extended attribute
  removexattr   remove an extended attribute
  lgetxattrs    \
  lsetxattr     (same as above, but operate on symbolic links)
  lremovexattr  /

See attr(5) for more information.

This also adds support for the FBuffer field type, which maps to
an XDR opaque<> or a C (int, char *) pair.