Make flag strings clear specified flag with 'no' prefix

Remove flagmask field from set_mapgen_params table
Add small bits of needed documentation
Tune block emerge and sending parameters to more aggressive values
Merge commit '83bafbe' into merge2

Conflicts:
	src/mapgen_math.cpp
	src/util/string.cpp
Merge commit '5771052' into merge2

Conflicts:
	freeminer.conf.example
	src/defaultsettings.cpp