Control socket for DB services does not contain PID breaking use of relative name for -t argument

[Frode Nordahl]

Both the `ovs-appctl` and `ovn-appctl` programs will construct an absolute path to control socket when provided a relative name to the `-t` argument.

They do so by looking up the PID through a named PID-file [0], expecting the control socket filename to contain the PID [1].

At present the ovn-ctl script specifies the full path for the ovsdb-server control socket [2], presumably because `ovsdb-server`'s default behavior is to create one named after the `ovsdb-server` program [3], and having multiple of those may be confusing (?).

However, this does break the use of `ovn-appctl -t` for the DB sockets.

Would it perhaps be better to not set the control socket name, and allow the lookup through the named pid-file work as expected?

0: https://github.com/ovn-org/ovn/blob/4a77767e624fb735bbce3cb4e2dca9da92c418f4/utilities/ovn-appctl.c#L215
1: https://github.com/ovn-org/ovn/blob/4a77767e624fb735bbce3cb4e2dca9da92c418f4/utilities/ovn-appctl.c#L221
2: https://github.com/ovn-org/ovn/blob/4a77767e624fb735bbce3cb4e2dca9da92c418f4/utilities/ovn-ctl#L279
3: https://github.com/openvswitch/ovs/blob/2f34475a9708617eaa484044a5b485980b734b38/lib/unixctl.c#L237