connection: add syncronous message reply mechanism

connection: add syncronous message reply mechanism

Sending a message with (KDBUS_MSG_FLAGS_EXPECT_REPLY |
KDBUS_MSG_FLAGS_SYNC_REPLY) will result in the KDBUS_CMD_MSG_SEND ioctl
will block until the receiving connection has replied or the specified
timeout is hit.

When the round-trip succeeded, .offset_reply in the sender's message is
set to the offset where the reply can be found, and the message is
accepted and received automatically. The sender is still in charge of
freeing the message though.

This breaks with Greg's original matra that we followed elsewhere
in the design of kbus: "nothing blocks, ever". But hey, it an optional
feature after all :)