Fix *xattr prototype mismatches
Wrap all *xattr additions inside HAVE_ tests
Make t.xattr silently succeed on platforms without setcap/getcap
Disable xattr functions on __APPLE__ for now

The eight supported *xattr functions on __APPLE__ all have
different prototypes from the same-named Linux versions and will
require more changes to be supported properly.
Merge remote-tracking branch 'mackyle/pull-request' into upstream