daemon: xattr: simplify the enabling of the linuxxattrs features

Instead of enable them when having one of the two headers for it but
still checking for the HAVE_* availability of each *xattr() function
used, just enable the linuxxattrs as a whole when having any of the
needed headers (like before) and all the needed functions.

This might cause the linuxxattrs to not be available anymore on systems
without the whole set of *xattr() functions implemented, but OTOH it
simplifies the xattr.c implementations.