Added extended packet sending and receiving semantics to TMXR allowing for an optional frame byte to exist between length prefixed data packets
DMC11 Rework

- Reworked the pdp11_dmc implementation to use the TMXR library for network I/O.
- Removed the need for a Primary or Secondary mode describing a DMC line setup/operation.
- Added additional unit to perform I/O polling activities to the DMC and DMP/DMV devices.
- Added Modem status information describing the link state to the register information provided.
- Added a command out queue to pass status information to the host.