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

[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.
	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).

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