Re-worked the patchset, mostly to avoid having scattered changes to the same
areas, trying to keep each "commit" on a narrower focus/topic.
Fixes#10211, and attempts to fix some remaining _INSTALL_SCHEMES issues.
(makes `platinclude` point to the system headers for the default scheme,
which should avoid having to set HAIKU_USE_VENDOR_DIRECTORIES=1 when bulding
gdb).