screen: Get process arguments without /proc on FreeBSD and OpenBSD

FreeBSD and OpenBSD systems usually don't have /proc mounted, so it is
better not to depend on it when reading process arguments.

The code for FreeBSD also works on DragonFlyBSD although it seems
DragonFlyBSD have /proc mounted by default.

https://bugzilla.gnome.org/show_bug.cgi?id=750494