If this happens the imsg may no longer be usable as there may be queued
messages, but this is a) already the case with the code now, and b)
would be the case if recvmsg() fails anyway, so we can document that -1
from imsg_read() invalidates the struct imsgbuf.
discussed with and ok eric
client-server protocol more resilient and make the protocol versioning work
properly. In future, the only things requiring a protocol version bump will be
changes in the message structs, and (when both client and server have this
change) mixing different versions should nicely report an error message.
As a side effect this also makes the code tidier, fixes a problem with the way
errors reported during server startup were handled, and supports fd passing
(which will be used in future).
Looked over by eric@, thanks.
Please note that mixing a client with this change with an older server or vice
versa may cause tmux to crash or hang - tmux should be completely exited before
upgrading.
