ipc: implement ipc for bml<->bmlhost messaging

Implement a simple unix domain socket based ipc. Use it for a few commands. Use
a string pool to handle commands that return pointers to static string.
info: don't print real values for props/attrs that returned errors before

When we don't get meta-data for properties/attributes don't try to get the
'real value' either.
bmlipc: implement a few more commands

Also use shutdown() instead of close() on the client. Turn the #ifdef for the
manual debug mode into a runtime check using another env-var.