rec: actively deprecate old style settings

[omoerbeek]

Short description

Disable old-style parsing by default. Enable (on the command line) with --enable-old-style.
Probably will cause some grumblings, but so be it.

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)

[coveralls]

Pull Request Test Coverage Report for Build 11663936347

Details

• 3 of 6 (50.0%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.004%) to 64.643%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/recursordist/rec_channel_rec.cc	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/recursordist/rec-tcpout.cc	2	63.49%
pdns/recursordist/lwres.cc	2	69.66%
modules/lmdbbackend/lmdbbackend.cc	2	73.53%
pdns/rcpgenerator.cc	2	89.59%
pdns/recursordist/settings/cxxsettings-generated.cc	2	91.62%
pdns/iputils.cc	3	55.91%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/recursordist/syncres.cc	3	79.48%
pdns/recursordist/recpacketcache.hh	3	89.55%

Totals
Change from base Build 11663498636:	0.004%
Covered Lines:	124779
Relevant Lines:	162252

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💛 - Coveralls

[omoerbeek]

Rebased to fix conflict in upgrade.rst