Generalize the {{cam=}} argument to iic commands

The cam= option to iic exposure commands currently takes a list of camera names. For the benefit of Gen2 programmers, we'd like to generalize that to describe common sets of cameras:

• all – all available/valid cameras. Equivalent to having no cam= argument.
• b,r,m,n – all available/valid cameras of that arm
• smN – all available/valid cameras on that module