dnsdist-1.9.x: Backport 14569 - Fix EDNS flags confusion when editing the OPT header

[rgacogne]

Short description

Backport of #14569 to dnsdist-1.9.x

We used to wrongly reverse the byte-ordering of the existing EDNS flags when editing the OPT header, for example when setting an extended DNS error status.

(cherry picked from commit 010521a)

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 10775159815

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• 10 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.009%) to 55.784%

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Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-async.cc	2	82.21%
pdns/dnsdist-tcp.cc	8	75.83%

Totals
Change from base Build 9987810845:	-0.009%
Covered Lines:	47995
Relevant Lines:	65509

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💛 - Coveralls