pothos-library: packet label index in-terms of element size

Redocumented the Packet definition for labels to have indexes related to the payload dtype.
And adjusted the packet to and from stream block to adjust the index for the payload dtype.
pothos-blocks: round down mtu to elem multiple
pothos-blocks: network endpoint open/close requires buffer

The buffer cannot be non-empty, we need to be able to drain
data from runtime to be able to perform the handshake.
Size zero buffers will never drain the partial packets.
So the handshake gets stuck...
pothos-blocks: network endpoint use chrono, lower def timeout
pothos-blocks: added optional flags to socket endpoint calls

This fixes a possible error when recv'ing a header,
we now use MSG_WAITALL to guarantee that the complete header is waited on.

In addition, MSG_MORE is used to hint to send that additional sends will be called.
This enables the underlying socket guts to make better decisions about sending.
pothos-blocks: MSG_MORE does not belong on this send()
pothos-blocks: thread safety work for network sink