[ics_xcu_pcm] Make some/all PCM commands synchronous, returning values before finishing

This issue was initially filed into JIRA by 'cloomis' at 01/Oct/15 6:45 PM as ICS-19.
This issue was assigned to 'Steve Hope'.

The PCM is roughly untouched from previous projects, in which all status output was via periodic UDP packets. This is not a good match for the PFS system, where command+response is natural.

For some things it does not matter much; for others it causes trouble. In particular, we would like the IO bitmask of enable power ports to be generated when a power change command is done, and for all motor keywords to also be available right away.

Note that there a couple of ways of doing this. Adding a command for retrieving some/all of the available values would be sufficient.

I'll call this important for when the focus stage needs to be properly controlled.