Enable library grade detection by default.

Use -R instead of -r with cp by default.

Fix some documentation errors.

compiler/options.m:
	Enable library grade detection by default.

	Document the --no-detect-libgrades option.

	Make the default install command dir option, -R,
	rather than -r.  The former is more portable.

scripts/Mercury.config.in:
	Do not hardcode the set of library grades at configuration time.

compiler/mercury_compile.m:
	Do not clear the library grade set before adding any detected
	grades since doing so also prevents additional grades being
	specified via the LIBGRADES variable.

doc/user_guide.texi:
	Document --no-detect-libgrades.

	Replace some uses of `:' as a module qualifier.

	Fix a typo: s/outut/output/.