posix shell: allow to run multiple instances

Tools/sitl_multiple_run.sh

cmake/posix/px4-alias.sh_in

launch/single_vehicle_spawn.launch

platforms/posix/cmake/px4_impl_os.cmake

platforms/posix/src/main.cpp

platforms/posix/src/px4_daemon/client.cpp

platforms/posix/src/px4_daemon/client.h

platforms/posix/src/px4_daemon/pipe_protocol.cpp

platforms/posix/src/px4_daemon/pipe_protocol.h

platforms/posix/src/px4_daemon/server.cpp

platforms/posix/src/px4_daemon/server.h