script: add 'Script started' line always to capture file

The scriptreplay(1) will expect the capture file to have a header that is
omited.  Before this change the --quiet option together with timing
caused following replay error.

$ script --quiet --timing=timing
[...]
$ scriptreplay timing typescript
[...]
scriptreplay: unexpected end of file on typescript

Signed-off-by: Sami Kerola <kerolasa@iki.fi>