Receive new data into buffer at context->m_receiveOfs.

Without this, previously received partial message waiting in buffer
will be overwritten when the rest of the message arrives.
Bail out of packet eater if less than 4 bytes available.

Without this, packlen will be initialized with undefined data. This
causes "oversize packet" errors.
Fix mixed line ending style in usynergy.h

In the absence of any guidence in the coding style docs, convert it
to use CRLF throughout. That's what the other usynergy sources do.
Implement DMRM message for relative mouse movement.