+ Improve related comments.
Registry hives are opened in shared read access when NT is loaded in PE
mode (MININT) or from network (the hives residing on a network share).
This is true in particular for the main system hives (SYSTEM, SOFTWARE,
DEFAULT, ...).
However, in PE mode, we can allow other hives, e.g. those loaded by the
user (with NtLoadKey) to be loaded with full read/write access, since we
boot from a local computer.
feb67576dd