Lock access to conf/incoming file

With really bad timing we might run into something like this:

| + sudo reprepro -v -b /srv/repository/release/release-42-rc1 --waitforlock 1000 --ignore=wrongdistribution processincoming release-42-rc1 foo_23.1_amd64.changes
| Multiple definitions of 'release-42-rc1': first started at line 2 of /srv/repository/release/release-42-rc1/conf/incoming, second at line 11 of /srv/repository/release/release-42-rc1/conf/incoming!

Use flock(1) from util-linux to lock access to the
conf/incoming file.

Closes: MT#4751
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