sysdb: Let sdb_command_print_reply() accept client instead of input objects.

This is more flexible.
core: Allow objects to have the same size as sdb_object_t.

That is, allow minimalistic objects which only have a name.
sock.h: Fixed documentation of sdb_client_send() return values.
sysdb: Added support for the -c <cmd> option.

This option (which may be specified multiple times) may be used to send
commands to the server in a non-interactive way. After handling the reply from
the last command, the process exits.
frontend: Close a connection when receiving an exception of the fd.
frontend: Be more robust in case of failed connections.

Make sure not to write to a failed connection, most notably, don't try to log
to a failed connection.
sysdbd: Ignore SIGPIPE (e.g. when a client connection was aborted).