FindImageMagick: Find libraries named with HDRI support (#14348)

When ImageMagick is built with HDRI support, the library names get a
HDRI suffix.
FindTCL: Add BSD paths for Tcl/Tk 8.6

Follow-up to commit 4a015f77 (OpenBSD: Add paths for Tcl/Tk 8.4/8.5,
2012-12-03): those paths added for OpenBSD also work on other BSDs, some
of which are already using version 8.6 of Tcl/Tk.
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 'FindTCL-paths-for-8.6'

141ce01 FindTCL: Add BSD paths for Tcl/Tk 8.6
Merge topic 'FindImageMagick-hdri'

5803b94 FindImageMagick: Find libraries named with HDRI support (#14348)
Merge topic 'fix-install-destinations'

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