Eliminate the fallback strndup() and strnlen() implementations.

Cast to integer after multiplying with 1000.  Allows sub-second
seeking.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Save the state file 2 minutes after the last change.  This reduces the
disruptions by an idle MPD, and MPD can be paged out permanently until
it is used.

Only relevant for old ffmpeg versions.

Fixes valgrind warning.