Feature: daemons: execd should read remote client messages async.

Instead of calling pcmkread_remote_message, which blocks, use
pcmkread_available_remote_data instead. This function will attempt to
read a message but if the other side of the connection isn't ready or
doesn't give us the complete message, we'll go back to the main loop and
try again later instead of block.

Additionally, clean up the code formatting a little bit and make the
error messages more helpful.

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