IcedTeaWeb support for openjdk 21+

[bourgesl]

Since jdk18 deprecated java SecurityManager methods, icedtea-web fails to start any jnlp app.

I propose to make icedtea-web work with latest jdk in degraded mode ie less-secure as missing security manager features will just say Yes (like full-privileges).

I will make this new BypassSecurityManagerSupport in my icedtea-web 1.8 fork as I want to make my own ( jdk21 LTS + icedtea-web 1.8.x ) bundle for my end-users.

PS: Hacking freedom is amazing !

Cheers, Laurent