SCP: Various cleanups.

- Avoid assignments of void * values.  Cast all memory allocation return values to appropriate types.
- Add output to sim_log where missing in various places.
- Fixed issue with lost file positions after a restore for devices which leverage the UNIT_SEQ flag.
TMXR: Packet, Loopback and Half-Duplex enhancements

- Added halfduplex mode for network connections and corrected modem signal DSR to reflect connection status (vs original attach status), and DCD follows DSR (except in halfduplex mode where it follows CTS).

- Enhance tmxr_set_get_modem_bits to also return the modem DTR and RTS state.

- Separate RTS from DTR when manipulating modem state bits

- Minor fixes to loopback functionality after direct testing with the first loopback client device (DMC11).

- Fix clearing of break input buffer now that input buffers are dynamically allocated

- Changed Modem bit logic to have CTS reflect RTS as expected by devices which may expect this.

- Changed receive buffers to be dynamically allocated and the same size as transmit buffers when transmit buffers are non-default sized.

- Added TMXR line attach in loopback mode.  Fixed loopback buffer management

- Added loopback support to TMXR lines

- Added functioning connect poll capability to revised DMC

- Added connection destination display to connection status even when a connection has not yet been established.

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