pakfire: Prevent from get launched multiple times.

pakfire: Prevent from get launched multiple times.

When pakfire gets launched a check if a so called lockfile exists and
the process will be aborted, otherwise the file will be created which
prevents any other pakfire instance to perform any operations until the
first process gets finished and the lock will be released again.

Because the release of the lock is located in an END block, the lock
also will be released in case the pakfire process gets interuped or
gains an error.

This prevents from an lock loop and an unuseable pakfire.

Reference: #12621.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>