spci: Terminate all commands with a linefeed for all transports

While some transports add a terminating (carriagereturn+)linefeed
unconditionally, the USBTMC transport does not. At least the R&S HMO1002
requires the linefeed and locks up otherwise. Fixes bug #784.

This changes the TCP and VXI transport from CR+LF to LF only.

Also fixes a possible memory leak for VXI, where the temporary command
buffer was not freed in case of a write error.