Fix rotation condition when SPDLOG_NO_DATETIME is defined.

When SPDLOG_NO_DATETIME is defined, the rotation was never worked
because `msg.time` is always zero.

Fix deprecation warnings in filename_to_str and improve performance of wbuf_to_utf8buf

Provide source location support for systemd_sink.h

Fix missing include for null_mutex in rotating_file_sink.h

Fix warning: redundant move in return statement

Fix use of incomplete type in stdout sinks