Add PIE and RELRO to binaries

doveadm replicator add <user mask> command added. This allows quickly adding users from userdb that don't exist in replicator. For example "doveadm replicator add '*'" adds all the new users from userdb. This isn't really necessary though. Users will be automatically added as soon as their mailboxes change in some way and replicator gets notified about them.

replicator: Added doveadm socket for communicating with "doveadm replicator".

replicator: Renamed doveadm-connection to dsync-client.

MODULE_LIBS is now part of LIBDOVECOT. No need to explicitly link it everywhere.

Initial implementation of dsync-based replication.