src/daemon/plugin.[ch]: Use cdtime_t for the interval argument of "register complex...

src/daemon/plugin.[ch]: Use cdtime_t for the interval argument of "register complex read".

All plugins converted to "struct timespec", just so that
plugin_register_complex_read() would convert back to cdtime_t again. This patch
removed this crazyness; the new "determine automatically" value is zero (used
to be NULL).