Mgv5/v7/fractal: Add 'large_cave_depth' parameter to replace fixed value

The value cannot be fixed because we can shift terrain vertically.
This also makes these mapgens consistent with mgflat and mgvalleys which
have 'large_cave_depth' parameters.

minetest.conf.example

@@ -1256,6 +1256,10 @@
 #    type: float
 # mgv5_cave_width = 0.125
 
+#    Y of upper limit of large pseudorandom caves.
+#    type: int
+# mgv5_large_cave_depth = -256
+
 #    Y-level of cavern upper limit.
 #    type: int
 # mgv5_cavern_limit = -256
@@ -1371,6 +1375,10 @@
 #    type: float
 # mgv7_cave_width = 0.09
 
+#    Y of upper limit of large pseudorandom caves.
+#    type: int
+# mgv7_large_cave_depth = -33
+
 #    Controls the density of floatland mountain terrain.
 #    Is an offset added to the 'np_mountain' noise value.
 #    type: float
@@ -1522,6 +1530,10 @@
 #    type: float
 # mgfractal_cave_width = 0.09
 
+#    Y of upper limit of large pseudorandom caves.
+#    type: int
+# mgfractal_large_cave_depth = -33
+
 #    Choice of 18 fractals from 9 formulas.
 #    1 = 4D "Roundy" mandelbrot set.
 #    2 = 4D "Roundy" julia set.