Improvements/fixes for noise parameter input in advanced settings

Improvements/fixes for noise parameter input in advanced settings

Formspec input for each individual noise parameter and flag.
Allow noise flags to be set in advanced settings, previously only settable
in minetest.conf.

Standardise 'group format' for noise parameters set in minetest.conf, as
only these support noise flags. However the older 'single line' format is
still accepted to support existing minetest.conf files.
Therefore auto-generate minetest.conf.example with noise parameters in
'group format'.

Setting 'type' in settingtypes.txt is now either 'noise_params_2D' or
'noise_params_3D', the dimension number is displayed in the advanced
settings edit page.