Simplify set_server_start_time().

Simplify set_server_start_time().

This function receives a pointer to a timeval structure which is
supposed to contain the daemon startup time. Passing NULL means to
set the daemon startup time to the current time.

There are only two callers of this function, in audiod.c and in
server.c.  The first one passes NULL, the other one initialises the
timeval structure pointed to by the global now pointer to the current
time, and passes this pointer.

It's easier to let set_server_start_time() always act as if NULL had
been passed.