Prevent multiple instances of Hive to run at the same time

[christophe-calmejane]

Part of code already written in main.cpp, but it's not complete! It's imperative to check for the runnnig pid, or a crash will lock Hive until reboot on macOS (shared memory not properly released by the OS when the last instance is killed)

[christophe-calmejane]

Maybe take a look at https://github.com/itay-grudev/SingleApplication

[christophe-calmejane]

Or use Fastbuild SystemMutex class: https://github.com/fastbuild/fastbuild/blob/main/Code/Core/Process/SystemMutex.cpp

[christophe-calmejane]

It might even be more useful to ask the user if it wants to start Hive on another "port" if it's already running, but it means we have to keep a list of the running processes in a lock file, instead of just one.