Clean up install rules of CMake itself (#14371)

Ensure CMAKE_DATA_DIR, CMAKE_DOC_DIR, and CMAKE_MAN_DIR are always
relative paths in CMake code, and set defaults accordingly.  Use the
install() command instead of install_files() and install_targets().
This is more modern and also avoids stripping of the first character
from user-specified destinations.

While at it, fix the default destinations reported in the bootstrap
help.
Merge topic 'fix-install-destinations' into next

df62f64 Clean up install rules of CMake itself (#14371)