Since commit 2642f432ef (Aux: Install editor and bash files to more
natural locations, 2020-03-30, v3.18.0-rc1~429^2) these files are
installed by default into locations that are not CMake-specific but may
be distro-specific. Add options for packagers to control these
locations.
Also rename the `CMAKE_BASH_COMP_DIR` option to follow our conventions.
Fixes: #20993