Fix a memory leak in ts_dimm: AstelcoDIMM is accumulating command info in running_commands instead of purging done commands. This is exacerbated by the fact that status_loop is sending command far too often.

Improve the handling of invalid data from the Astelco DIMM. There are two issues.

• There are a number of values that mean "could not determine the value". One of those is LOCKEDBY, which we were seeing shortly after setting the mode. Handle that and all other error codes.
• A string variable with unknown value can be NULL (no double quotes) if unknown; handle that properly.