CMake: Improve config installation

Use 'GNUInstallDirs' standard module to set destinations of installed
files.

Use uppercase "CURL" names instead of lowercase "curl" to match standard
'FindCURL.cmake' CMake module:
* https://cmake.org/cmake/help/latest/module/FindCURL.html

Meaning:
* Install 'CURLConfig.cmake' instead of 'curl-config.cmake'
* User should call 'find_package(CURL)' instead of 'find_package(curl)'

Use 'configure_package_config_file' function to generate
'CURLConfig.cmake' file. This will make 'curl-config.cmake.in' template
file smaller and handle components better.  E.g.  current configuration
report no error if user specified unknown components (note: new
configuration expects no components, report error if user will try to
specify any).

Closes https://github.com/curl/curl/pull/2849