retargeting to 2.3.0

Version is 2.1.2; this is not available in the drop-down list.

The tcp_close() function returns an err_t, and the documentation says:

> ERR_OK if connection has been closed another err_t if closing failed and pcb is not freed

However:

• tcp_close always returns tcp_close_shutdown(pcb, 1)
• tcp_close_shutdown always returns either ERR_OK or tcp_close_shutdown_fin(pcb)
• tcp_close_shutdown_fin returns either ERR_OK or tcp_send_fin(pcb), except in the latter case, ERR_MEM is converted to ERR_OK
• tcp_send_fin returns either ERR_OK or tcp_enqueue_flags(pcb, TCP_FIN)
• tcp_enqueue_flags returns either ERR_OK or ERR_MEM

Therefore tcp_close can only ever return ERR_OK. Normally this wouldn’t matter much, and I would say it’s reasonable to check for errors in case future versions of lwIP need to add error returns, but tcp_close is special: closing and disposing of resources is something that’s often done in situations where recovering from an error is difficult (for example when already handling another error). It would make using lwIP more ergonomic if tcp_close were documented as never failing, which ideally would be accompanied by a change of signature to return void instead of err_t.