Harden serial comm interface

[Tails86]

This PR is fully compatible with DreamPort v0.999 and is compatible with older versions of DreamPort in host-1p mode.

1. I found an issue with the DreamConn controller not being deleted when unplugged and traced it to the shared_ptr held within maple_devs.cpp, so I created tearDownDreamConnDevices()
   • I was having trouble putting the right delete calls in there and eventually just switched MapleDevices to use smart_ptr
2. Renamed DreamcastControllerUsbPico to DreamPort
3. A single serial object is used for DreamPort devices. This still means that only 1 device is supported at a time, but now there is support for the host-4p mode which can communicate with 4 controllers at once.

@kosekmi helped me to test these changes. He reports they work on all OS, but we have a few minor bugs to work out in the future.

• MacOS and Linux: When disconnecting DreamPort during a game, the reconnect ends in "Operation timed out". Can only be solved by restarting Flycast
• Changing the bus number of a DreamPort device during gameplay does not change over the communication (because nothing is being done to communicate that with maple_devs.cpp)

There are also a few things I'd like to clean up with future PRs:

• I think it makes sense for the current "DreamConn" to be a virtual base class and do away with TYPE_DREAMCONN and TYPE_DREAMPORT macros
• I have some issues wrapping my head around DreamConnVmu and DreamConnPurupuru - the way it's swapping out what was already there. Seems like something should be injected into maple_cfg to allow the controller to dictate what gets created, but I don't feel confident with this code yet to say with much conviction.
• The _unique_id of the SDLGamepad can be adjusted to serial and known letter of a DreamPort device to overcome the weird randomness of Windows
• I want to continue working toward supporting multiple separate DreamPort devices

Thank you for allowing us to contribute to this project. It has been a great experience, and I love this emulator now 😄