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() -> msp_connect()
Rename msp_get_local_adr() -> msp_get_local()
Rename msp_get_remote_adr() -> msp_get_remote()
msp_set_local() takes (const struct *) arg2
msp_connect() takes (const struct *) arg2
void msp_set_local(), msp_connect(), msp_listen(), msp_set_handler()
Improve MSP API documentation