libroot_build: Properly remap fs_attr_* functions.

This was done using macros before, which isn't the way we have things set up.
In theory that method should work, however if not all consumers include the
libroot_build headers properly, then it breaks in subtle but confusing ways,
which is not what we want at all.

Thanks to Jessica for advice.

Change-Id: Idd45df5547daecf8239932957088da03ddfccf87

libroot_build: Full passthrough to system attributes.

As it turns out, using the xattr emulation layer plus "libgnu"
causes some strange mixups at package build time, and so packages
built with it were winding up with no attributes at all.

So I've just bitten the bullet and written a full passthrough layer
to the system attributes. Verified using a full build of haiku.hpkg
this time ... after a lot of painful debugging of symlink mixups.

Hopefully I am finally rid of this plague...