kernel32: implement QueryFullProcessImageFileNameW

[Mikołaj Zalewski]

  This patch builds on a bug in ntdll - 
NtQueryInformationProcess(ProcessImageFileName) returns an NT path on 
Windows, not a DOS path as this happens in Wine. However, fixing this is 
not trivial (this path comes from the loader that uses DOS paths. The 
path should start with '\Device' and I've mainly seen '\??\' paths in 
our code - I don't know how well we support the formers), and after 
fixing it, implementing QueryFullProcessImageFileName would require 
RtlNtPathNameToDosPath_U that is not implemented. As ntdll is not used 
directly often and the current code probably worked for the one who 
implemented it, I hope keeping it like that is ok - I have only added 
comments about it.