usermode calls from kernelmode

During my development of some tests for ZwAllocateVirtualMemory the need to call NtQueryVirtualMemory arose but unfortunately it turned out this syscall is not exported for use in drivers. As such after a discussion with thfabba I ended up implementing this initial prototype of making usermode calls from kernel mode. Currently only NtQueryVirtualMemory is supported but I think the code can serve as a skeleton for further enhancements in that direction.