CORE-13951
MIDIMAP_LoadSettings and PlaySound:
Use RegOpenCurrentUser to access the appropriate HKEY_CURRENT_USER key.
This allows functions that use HKEY_CURRENT_USER to be called from system
processes while impersonating, for example: winlogon.
PlaySound:
- Open the correct registry key.
- Expand environment strings if registry value type is REG_EXPAND_SZ.
- Handle "SystemDefault" event and improve flags handling.
- Don't use proc_PlaySound as thread proc.
- Fix SND_ASYNC flag for system processes.
Move get_mmio* code out of proc_PlaySound to fix impersonation problem.