build: refactoring CMake and ns3

Includes:
- summarize ns3 commands on top-level --help
- add --quiet as a post-positional argument
- refactor verbose variable names
- aggregate ./ns3 --check-config|profile|version into ./ns3 show config|profile|version
- remove ns3 --check deprecated option
- set VERBOSE environment variable when building/running with -v/--verbose
  https://gitlab.com/nsnam/ns-3-dev/-/issues/590#note_870520212
- enable verbose Makefiles when NS3_VERBOSE is enabled
  https://gitlab.com/nsnam/ns-3-dev/-/issues/590
- introduce default build profile and refactor others
  https://gitlab.com/nsnam/ns-3-dev/-/issues/591
- use "-Og" in "CMAKE_BUILD_TYPE=Debug"/"ns3 -d debug"
- add back FindPython3 and fallback to FindPythonInterp if needed
- redirect pybindgen apiscan output to apiscan.log
- enable CMAKE_FIND_DEBUG_MODE with NS3_VERBOSE and CMake >= 3.17
- add search path logging to find_external_library
  Requires NS3_VERBOSE=ON. This is an anternative to CMAKE_FIND_DEBUG_MODE=true available in CMake >= 3.17
- remove C support
- reduce Int128 checks
- fuse Boost Units Quantity and SI header checks
- replace not found messages with skipping

CMakeLists.txt

@@ -19,7 +19,7 @@ endif()
 # ##############################################################################
 # Project name                        #
 # ##############################################################################
-project(NS3 CXX C)
+project(NS3 CXX)
 
 file(STRINGS VERSION NS3_VER)