Fix mapgen settings in minetest.conf being ignored

[sfan5]

broken since e8a8185

To do

This PR is a Ready for Review.

How to test

#9296 (comment)

[paramat]

Thanks will test.
Serious bug.

[SmallJoker]

#              v-- MGV7_MOUNTAINS | MGV7_RIDGES | MGV7_CAVERNS
# defaultsettings: mountains,ridges,nofloatlands,caverns
#  + Global .conf: nomountains, noridges, floatlands
#  + Game   .conf: nomountains, ridges, nocaverns
#
#          wanted: nomountains, ridges, floatlands, nocaverns
#
# ==> Actual map_meta.txt value:
# mgv7_spflags = nomountains, ridges, floatlands, nocaverns


# minetest.conf:
mgv7_spflags = nomountains, floatlands


# game/minetest.conf:
mgv7_spflags = nomountains, ridges, nocaverns

Works as expected. So far.

-- map_meta.txt: nomountains, ridges, floatlands, nocaverns

print(dump(minetest.get_mapgen_setting("mgv7_spflags")))
-- "nomountains, floatlands" (first join)
-- "nomountains, ridges, floatlands, nocaverns" (second join)

This is so confusing but works at least better now.

[paramat]

minetest.conf:
mgv7_spflags = nomountains, floatlands

Note that this is a discouraged and sloppy way to set flags in .conf, because every flag should always be stated, with or without a preceding 'no'. The presence, or lack of, a preceding 'no' is what sets the value, not whether the flag is present.
It is better that a user states all flags so that it is clear what they are choosing.
Also, when using 'All Settings' menu, the resulting setting in .conf states all the flags.

However, it seems that we now have the proper fallback logic that, if a flag is not stated, it is not altered from the default.

[paramat]

Confirmed the bug in master, surprised and ashamed i did not notice this bug earlier.
In master:
Global mapgen flags set in .conf work fine.
Mapgen-specific flags set in .conf are ignored.
Other mapgen-specific settings, like mgv7_cave_width set in .conf work fine.

[paramat]

Tested PR, mapgen-specific flags set in .conf are now applied.
I do not understand the code here, but if SmallJoker considers the code ok i think it should be merged.