Keep a list of which search paths are re-rootable.

In preperation for the new RootPathSystemOnly mode, keep an
auxiliary list of which paths can be re-rooted and which can't.
This involves maintaining a boolean vector that is always modified
in sync with the search paths. Without the system-only mode, all
paths are re-rootable.
Add a new SYSTEM_ONLY mode for CMAKE_FIND_ROOT_PATH.

This new SYSTEM_ONLY mode allows for the intrinsic system paths
used for find_* commands to get re-rooted from the CMAKE_FIND_ROOT_PATH
variable while maintaining original user-specified paths.  The intent is
for this to be the preferred mode for cross compiling.  It allows for a
a restricted default search within a desired system root but also
lets the user specify explicit search paths anythere on the host system.
Merge topic 'add-find-root-system_only-mode' into next

e40ba091 Add a new SYSTEM_ONLY mode for CMAKE_FIND_ROOT_PATH.
881f4e6c Keep a list of which search paths are re-rootable.