[FREELDR] Improvements for GDT descriptors initialization.
Triggered by a fix attempt by M.Harmstone in PR #1905.
- Introduce #defines and helper functions so as to initialize the
descriptors in a human-readable and tractable way, without magic values.
- Rework some comments.
- Document what the changes are in (Longhorn and) Vista+.
Data has been obtained through using the WinDbg command: dg 0x00 0xFF
(for example), that lists all the selectors from 0x00 to 0xFF included.
See https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/dg--display-selector-
for more details.