doveadm: Added support for mail commands to read an input stream (from stdin) This is done by calling doveadm_mail_get_input() from the command's init() function. Currently it reads the entire input into a seekable istream with hardcoded 5 minute timeout. The input stream sending works also through doveadm proxying. This could probably be used by dsync at some point to support proxying over doveadm proxies, but that would require some more work. Especially a flag for commands to specify that they allow non-blocking input streams.

doveadm: If we get disconnected from server, log the reason.

doveadm: Pass through the exit code from doveadm-server to client.

dsync: Added tcps (TCP+SSL) target.

dsync: Use iostreams instead of fd when receiving connection from doveadm-server.

dsync: Added tcp:host[:port] target for syncing via doveadm-server.

Make sure we check all the functions' return values. Minor API changes to simplify this. Checked using a patched clang that adds attribute(warn_unused_result) to all functions. This commit fixes several error handling mistakes.

doveadm: Handle -NOUSER replies from doveadm-server.

doveadm: Added client/server architecture support for running mail commands. This is done when doveadm_worker_count is non-zero.