allow runtime args to be used in command input

A command starting with '-' is processed as though it were an arg
specified at runtime.
fixed HPIB/Ethernet problem, added HPIB mode args

Reorganized the HPIB/Ethernet code and fixed a long-standing problem
where the 'ST9' HPIB command would hang after printing the first
result. Added arg '-hpib-hard' et al to select use of hardware HPIB
over Ethernet emulation rather than #ifdefs.