safe_strcpy() includes an unnecessary conditional branch

A code contribution introduced a wrapper safe_strcpy() for the standard library function strncpy(). This wrapper is checking whether the output was truncated even in cases where the caller does not care about it.

I think that we need a separate function for those few occurrences where the caller cares about truncation.

While I was investigating this, I noticed that the program client/mariadb-plugin is apparently not covered by the default test suites of our mtr tool.