Refactor default CMAKE_BUILD_TYPE to top level

Tidy up Config.cmake further, moving selection of default build type
for single mode generators to one of the first tasks in the top level
script. Add protection against setting build type when
CMAKE_CONFIGURATION_TYPES is set (which indicates a multimode
generator).
Refactor setting of INSTALL_MISC_DIR to top level

Move setting of variable locating path for installing architecture
independent files into top level CMake script for clarity and
convenience. Hard code its value to "share/camp" on all platforms as a
reasonable default.
Remove obsolete CMake Config module

With refactoring of functionality, Config.cmake is simply an aliased
include of the MacOSX.cmake module. Remove the Config.cmake file, and
replace include of it in top level CMake script with the include of the
MacOSX.cmake module.