META_MODE: Fix staging not respecting _DIR overrides.

This fixes atf-c.h not properly being installed to /usr/include/ (in
the stagedir) via its override of 'INCSDIR_atf-c.h= ${INCLUDEDIR}'.

This fixes building things that depend on atf.

Staging seems to ignore OWN/GRP/MODE settings and needs further exploration.

Sponsored by: EMC / Isilon Storage Division