part of minetest_game

spawned automatically by the default flowers code.

took all other plants out of flora group so they don't get spawned by that
code.

the cause of a weird facedir bug, which was actually the game doing it, not
plants_lib

abms will avoid the defaults and vice-versa, to avoid overgrowth.

default flowers mod handles that now.

abms to account for that (as default flowers are flora=1, not flowers=1).

my flowers mod, and renames it to flowers_plus.

backward-compat aliases, in preparation for a subset of these flowers
being added to "common".

also default debug to false again

oversight in the engine that in turn revealed a bug in my code)

plants_lib/init.lua:83: bad argument #1 to 'getn' (table expected, got nil)

better way to set default biome params, optimized-out some copypasta, thanks
to thexyz for spotting these.

deleting nodes surrounding the spawn target during the mapgen code, replacing
the spawn target, and checking the type of node directly below the spawn
target.

every time an ABM or on_generated is called.  Optimized a number of nil
comparisons ("if x == nil" -> "if not x" and similar).

plants_lib's calls to do something extra when a tree is generated at mapgen
time or spawned via the growing code.

Thanks to paramat for spotting this!