Cavegen/Mgv5/Mgv7: Add optional giant caverns

Add to MapgenBasic for use by multiple mapgens.
Add to mgv5 and mgv7, enabled by default.

Similar to mgvalleys caverns but half the scale.
Parameters for upper y limit, distance caverns taper to full size, and
noise threshold (full cavern size).
As with mgvalleys caverns are generated first and classic caves are
disabled in any mapchunk containing a cavern, to avoid excessive
spreading volumes of liquids.
This also avoids floating blobs of liquid where a large classic cave
has overgenerated out into a neighbouring previously-generated mapchunk.

minetest.conf.example

@@ -1174,6 +1174,18 @@ server_side_occlusion_culling = true
 #    type: float
 # mgv5_cave_width = 0.125
 
+#    Y-level of cavern upper limit.
+#    type: int
+# mgv5_cavern_limit = -256
+
+#    Y-distance over which caverns expand to full size.
+#    type: int
+# mgv5_cavern_taper = 256
+
+#    Defines full size of caverns, smaller values create larger caverns.
+#    type: float
+# mgv5_cavern_threshold = 0.7
+
 #    type: noise_params
 # mgv5_np_filler_depth = 0, 1, (150, 150, 150), 261, 4, 0.7, 2.0
 
@@ -1189,6 +1201,9 @@ server_side_occlusion_culling = true
 #    type: noise_params
 # mgv5_np_cave2 = 0, 12, (50, 50, 50), 10325, 4, 0.5, 2.0
 
+#    type: noise_params
+# mgv5_np_cavern = 0, 1, (384, 128, 384), 723, 5, 0.63, 2.0
+
 #    Noise parameters in group format, unsupported by advanced settings
 #    menu but settable in minetest.conf.
 #    See documentation of noise parameter formats above.
@@ -1284,6 +1299,18 @@ server_side_occlusion_culling = true
 #    type: int
 # mgv7_shadow_limit = 1024
 
+#    Y-level of cavern upper limit.
+#    type: int
+# mgv7_cavern_limit = -256
+
+#    Y-distance over which caverns expand to full size.
+#    type: int
+# mgv7_cavern_taper = 256
+
+#    Defines full size of caverns, smaller values create larger caverns.
+#    type: float
+# mgv7_cavern_threshold = 0.7
+
 #    type: noise_params
 # mgv7_np_terrain_base = 4, 70, (600, 600, 600), 82341, 5, 0.6, 2.0
 
@@ -1317,6 +1344,9 @@ server_side_occlusion_culling = true
 #    type: noise_params
 # mgv7_np_ridge = 0, 1, (100, 100, 100), 6467, 4, 0.75, 2.0
 
+#    type: noise_params
+# mgv7_np_cavern = 0, 1, (384, 128, 384), 723, 5, 0.63, 2.0
+
 #    type: noise_params
 # mgv7_np_cave1 = 0, 12, (61, 61, 61), 52534, 3, 0.5, 2.0