inspection: Refactor windows systemroot detection to allow re-use

This change refactors guestfs___has_windows_systemroot to
guestfs___get_windows_systemroot. The new function returns a
dynamically allocated char * which must be freed.

The new function is no less efficient than before, as it returns the
result of guestfs___case_sensitive_path_silently, which is required
anyway. The new code is slightly more efficient than before, as it
re-uses the result of this testing in guestfs___check_windows_root
rather than running it again.
inspect: Add internal match6 function
inspect: Partial support for non-standard windows system root

Support arbitrary windows system root for pre-vista systems where
boot.ini is on the same partition as the system root.
inspection: Reformat is_systemroot declaration.

Just code motion.