Use LLProcess::create() to launch SLVoice, rather than inline code.

This appears to close a long-pending action item, as it seems the original
LLProcessLauncher implementation was in fact cloned-and-edited from this logic
in LLVivoxVoiceClient::stateMachine().
In any case, leveraging LLProcess buys us:
- reduced redundancy; fewer maintenance points
- logging for both success and errors
- (possibly) better SLVoice.exe lifespan management.