Fix auto-selection of presets in GTK.

Fix auto-selection of presets in GTK.

In commit a7dc17c42 I apparently introduced two bugs in
changed_preset(). Firstly, the Custom menu option was being written
into the 'found' variable in nearly all cases, because it has a NULL
user-data pointer which caused it to take the wrong branch of an if
statement due to an erroneous complex condition. Secondly, having
written _something_ into 'found', I had set it to inactive rather than
active due to forgetting to change a FALSE into a TRUE.

Now when I start up Net with my usual nonstandard default parameters
(I like the 13x11 wrapping, so I set NET_DEFAULT=13x11w in my
environment), the right menu entry comes up ticked.