luci-lua-runtime: fix legacy luci-mod-rpc authentication

The legacy RPC module authentication fixup was improperly ported into the
Lua dispatcher stub module, rendering legacy RPC authentication broken.

Additionally, the split of the former `sysauth` cookie into `sysauth_http`
and `sysauth_https` ones was also imposed upon the RPC module, without
leaving the option to use just `sysauth` in place, so allow this cookie
name again for the legacy RPC logins.

Fixes: #6333
Fixes: e1932592 ("luci-base: use different cookie names for HTTP and HTTPS")
Fixes: ded8ccf9

 ("luci-base-ucode: add initial ucode based LuCI runtime")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>