Fix handling of legacy terminal types

Fix handling of legacy terminal types

Disable plugins/code requiring current terminal features not only on
"dumb" terminals, but on legacy ("not modern") terminals, too. This is
handled by using the axzsh_is_modern_terminal() function insetad of
axzsh_is_dumb_terminal().

And fix plugins to return 91 ("ignore") as result code in this case.

In addition, disable theming only on dumb terminals (as before), but
fall back to the standard "ax" theme on non-modern terminals, too!