MEDIUM: h2: centralize transfer of decoded frames in h2_rcv_buf()

MEDIUM: h2: centralize transfer of decoded frames in h2_rcv_buf()

We still call the parser but it should soon not be needed anymore. The
decode functions don't need the buffer nor the max size anymore. They
must also not touch the CS_FL_EOS or CS_FL_RCV_MORE flags either, so
this is done within h2_rcv_buf() after transmission.

The "flags" argument to h2_frt_decode_headers() and h2_frt_transfer_data()
has been removed since it's not used anymore.