gnrc_tcp: cleanup: various minor changes

sys/include/net/gnrc/tcp.h

sys/include/net/gnrc/tcp/config.h

sys/include/net/gnrc/tcp/tcb.h

sys/include/net/tcp.h

sys/net/gnrc/transport_layer/tcp/gnrc_tcp.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_eventloop.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_fsm.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_pkt.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_rcvbuf.c

sys/net/gnrc/transport_layer/tcp/internal/common.h

sys/net/gnrc/transport_layer/tcp/internal/eventloop.h

sys/net/gnrc/transport_layer/tcp/internal/fsm.h

sys/net/gnrc/transport_layer/tcp/internal/option.h

sys/net/gnrc/transport_layer/tcp/internal/pkt.h

sys/net/gnrc/transport_layer/tcp/internal/rcvbuf.h

tests/gnrc_tcp_client/Makefile

tests/gnrc_tcp_client/README.md

tests/gnrc_tcp_client/main.c

tests/gnrc_tcp_server/Makefile