build: unbreak attr_copy_fd() and attr_copy_file().

Mike's build-system-revamping commit 9ce21bd of January this year
accidentally dropped a crucial -include of "libattr.h". This silently
undefined the HAVE_F*ATTR macros, causing attr_copy_{fd,file}.c to think
that xattrs are not supported and to compile the functions in those
files into a 'return 0;' stub.

The code in these files uses the NLS _() macro, which was recently moved
into misc.h, which can only sensibly be included by things outside
libattr. So migrate the NLS macros into a new nls.h at the same time,
and include it from misc.h.

With these fixes in place, xattrs can be copied again.