Without the assert it is easy to run into suble bugs because
be_get_backend_param() was called before all arguments are set by
be_parse_arg().

This fixes opt/fehler314.c.

This fixes backend/long_double_arith.c.

This fixes backend/chordal2.c.

This fixes opt/boolopt3.c.

This makes them easier to read and avoids markdown interpreting __xxx__
as bold.

This plugs a memory leak.

Thus we only need to maintain the feature list once on the homepage.