Low: libcrmcommon: specify "scheduler" as scheduler IPC message protocol

... in IPC requests, instead of "crmd". This means that IPC replies will now
specify that protocol as well.

Since pacemaker-schedulerd IPC is not proxied to Pacemaker Remote nodes, any
client is guaranteed to have the same Pacemaker version as the server. That
means we can enforce the message type check on the client side.