socket targets may fail to bind to local socket when not provided

[goatshriek]

Socket targets can be opened with a local socket name provided, but this may also be set to NULL, in which case a local socket is generated (see the documentation of stumpless_open_socket_target for details). However, this socket is always the same, which means that if a target is not properly closed the local socket will remain. Successive targets will be unable to open, as they will attempt to bind to the same socket name and will fail as it already exists.

This could be fixed by adding some number of retries to the local socket name, ideally randomized so that the names are not linear. The solution should also be documented in the documentation, and of course tests for the fixed functionality should be created as well.