Many iocsh commands crash the IOC when not given a port

[exzombie]

Running, simply, asynEnable with no arguments crashes the IOC. Many commands do this. A partial fix is in #171; that PR introduces a new command, which is where I noticed that no existing commands check for a NULL port name. That PR fixes the connectDevice() function, which, in turn, fixes all iocsh commands that first connect to the port. But there are commands that still crash, such as asynInterposeEcho and friends.