package.bbclass: implement a failsafe strip in classes

package.bbclass: implement a failsafe strip in classes
 - package.bbclass now uses file-native and cross strip to reliably strip
   unstripped executables and check the return code.  For the moment a
   failure here doesn't cause the build to fail but does output a failure
   message direct to the controlling terminal (this is temporary).  This
   behaviour can be changed by forcing IGNORE_STRIP_ERRORS = ""
   The change works by using a new runstrip shell function for every
   potentially strippable file.  Since this calls native 'file' there is
   a dependency on all inheritors of package.bbclass for file-native and
   since this would break native builds (because of the cycle in
   file-native and it's own native depends) native.bbclass cancels this
   dependency (system file should be fine for native packages and, anyway
   native currently doesn't package.)