global: Rename client_connection_type to doveadm_connection_type

doveadm: Rename client_connection_type to doveadm_client_connection_type Move it to doveadm.h to allow plugins use doveadm-cmd.h

doveadm-server: Restructured client-connection code so that TCP and HTTP connection types are properly separated.

doveadm-server: Moved common client code back from client-connection-tcp.c to client-connection.c.

doveadm: Changed command contexts to contain the input, output, and connection type values directly. Before, it used a direct pointer to the connection. It used also flags to indicate the connection type, which is now consolidated in the connection type enum.

doveadm-server: http: Fixed crash occurring when disconnecting a client at server deinit.

doveadm-server: Show incoming connection's IP and running command is process title.

doveadm: Implement HTTP server API, slightly based on JMAP API