Misc flag fixes

[fekete-robert]

No description provided.

[MrAnno]

Sorry, let me clarify what we've discussed:

Both store-legacy-msghdr and dont-store-legacy-msghdr are available as general source flags, but the default is store-legacy-msghdr. This means setting dont-store-legacy-msghdr will change the default behavior, setting the other option does not make sense unless someone is afraid that we might change the defaults.

I think we should not remove store-legacy-msghdr from the doc, we could just state the default behavior instead.

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Python:
When configuring a Python source, both flags are available, but on the Python side, this is published as store-legacy-msghdr (there is no dont alternative).

This information is only useful in the writing server-style Python sources and in the writing fetcher-style Python sources chapter, but we haven't documented the flags field there yet.