Clean up and tweak build system

* Combine client and server man pages.
  * Update unit test options and available databases in man page.
  * Add `--worldname` to man page.
  * Fix a bunch of places where `"Minetest"` was used directly instead of `PROJECT_NAME`.
  * Disable server build by default on all operating systems.
  * Make `ENABLE_FREETYPE` not fail if FreeType isn't found.
  * Enable LevelDB, Redis, and FreeType detection by default.
  * Remove the `VERSION_PATCH_ORIG` hack.
  * Add option to search for and use system JSONCPP.
  * Remove broken LuaJIT version detection.
  * Rename `DISABLE_LUAJIT` to `ENABLE_LUAJIT`.
  * Rename `minetest_*` variables in `version.{h,cpp}` to `g_*`.
  * Clean up style of CMake files.

.travis.yml

@@ -3,9 +3,9 @@ compiler:
   - gcc
   - clang
 env:
-  - WINDOWS=32
-  - WINDOWS=64
-  - WINDOWS=no
+  - PLATFORM=Win32
+  - PLATFORM=Win64
+  - PLATFORM=Linux
 before_install: ./util/travis/before_install.sh
 script: ./util/travis/script.sh
 notifications:
@@ -13,7 +13,7 @@ notifications:
 matrix:
   fast_finish: true
   exclude:
-    - env: WINDOWS=32
+    - env: PLATFORM=Win32
       compiler: clang
-    - env: WINDOWS=64
+    - env: PLATFORM=Win64
       compiler: clang