Bug#35705: fvwm2: should register window manager fvwm2

[David Rocher <davroc@hplb.hpl.hp.com>]

package: fvwm2
version: 2.1.10-2

fvwm2.postint contains:
        system("register-window-manager --add icewm");
should be fvwm2 instead of icewm.

 David.