wineserver: Added support for read/only Registry, similar to unprivileged users (try 3)

[Paul Chitescu]

Changelog:
        wineserver: Added support for read/only Registry, similar to 
unprivileged users

Rationale:
        This is a first step towards implementing shared wine prefixes with 
most users being unprivileged and a single Administrator (power user, etc.) 
capable of installing programs.

How it works:
        For selected Registry files (system.reg and userdef.reg) the code will 
attempt to initially load them by opening the file in read/write mode.
        If opening R/W succeeds the current behavior is unchanged, the keys 
are created write accessible to the user.
        If opening R/W fails the operation is retried in R/O mode. If that 
succeeds the current key is flagged KEY_READONLY causing the entire loaded 
branch to inherit this flag.
        The create_key handler refuses to allow non-volatile keys to be 
created in a R/O parent key. However, if MAXIMUM_ALLOWED access is specified 
the access is silently downgraded to KEY_READ and the key is opened instead. 
If opening fails because the key doesn't exist report that access is denied 
(since it was supposed to create if possible).
        The open_key handler checks the access rights if the requested key is 
flagged R/O. If MAXIMUM_ALLOWED access is specified (Windows regedit.exe does 
so) the access is silently downgraded to KEY_READ. Else if any of the write or 
delete flags are set (KEY_DENIED) the request is refused with access denied.

This is pretty much what a Guest or unprivileged user gets when accessing the 
Registry on a Windows system (except we don't implement object ownership).

Changes from previous patch version:
- fallback to open_key in create_key handler if write access is not required, 
correctly set the error if failing to open
- can be applied with patch -p0 or -p1
- used option -p to diff

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