A future version of CMake may not print the "cmake version" line at the
beginning of the --help-*-list output. Filter out the line with 'grep'
instead of 'tail' to tolerate output from versions of CMake with and
without the version line. Match "cmake version", "cpack version", and
"ctest version" in each corresponding completion script.
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.