This implements the GameCube modem adapter. This implementation is stable but not perfect; it drops frames if the receive FIFO length is exceeded. This is probably due to the unimplemented interrupt mentioned in the comments. If the tapserver end of the connection is aware of this limitation, it's easily circumvented by lowering the MTU of the link, but ideally this wouldn't be necessary.
This has been tested with a couple of different versions of Phantasy Star Online, including Episodes 1 & 2 Trial Edition. The Trial Edition is the only version of the game that supports the Modem Adapter and not the Broadband Adapter, which is what made this commit necessary in the first place.
This expands the tapserver BBA interface to be available on all platforms. tapserver itself is still macOS-only, but newserv (the PSO server) is not, and it can directly accept local and remote tapserver connections as well. This makes the tapserver interface potentially useful on all platforms.
Martin Michelsen