explicitely in all paths,
- check locations given by the package installation script, and fail
if one is unknown,
- added "install-lib-version" extension, so that package installation
scripts can explicitely specify which version of install-lib they
need,
- added a message which is printed at the end of a successful
installation and indicates how to set SCSH_LIB_DIRS,
- provided a way to "turn off" boolean options like --force, by
providing an explicit "=no",
- replaced a few calls to "error" by calls to
"display-error-and-exit", to try to be consistent and use "error"
only for "internal" errors not due to the end user,
- removed "platform" parameter from layouts, not needed anymore since
the "lib" location was made platform-independent a long time ago,
- documented --no-user-defaults.
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