Improve MSP API

Replace struct msp_sock * with typedef MSP_SOCKET
Add new 'flags' arg to msp_socket(), unused for now
msp_send() returns ssize_t number of bytes sent; -1 for error
MSP handler returns size_t; number of bytes consumed
Add MSP_MESSAGE_SIZE definition
Rename msp_set_remote() to msp_connect()
msp_set_local(), msp_connect(), msp_listen(), msp_set_handler() all
return void