pakfire: Implement feedback from mailing list discussion

pakfire: Implement feedback from mailing list discussion

- Improve lockfile test: Return immediately if lockfile is present,
to prevent unnecessary and expensive "pidof" calls

- Add better explanation to the log file reading command and JS

- Change user interface: If no errors occurred, the page returns to
the main screen (after a short delay). If an error occurred, the log
output remains and a message is shown.

Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Acked-by: Peter Müller <peter.mueller@ipfire.org>