workstation: add nvidia_390 legacy driver class

Lain needs the legacy driver to function.

manifests/workstation/nvidia_390.pp

+#
+class freebsd::workstation::nvidia_390 {
+  file_line { 'Enable linux support':
+    path => '/boot/loader.conf',
+    line => 'linux_load="YES"',
+  }
+  ~> exec { '/sbin/kldload linux':
+    refreshonly => true,
+  }
+  -> package {
+    [
+      'nvidia-driver-390',
+    ]:
+      ensure   => installed,
+  }
+  -> file_line { 'Enable nvidia driver':
+    path => '/boot/loader.conf',
+    line => 'nvidia-modeset_load="YES"',
+  }
+  ~> exec { '/sbin/kldload nvidia-modeset':
+    refreshonly => true,
+  }
+
+  file_line { 'Fix broken vt-terminals':
+    path => '/boot/loader.conf',
+    line => 'hw.vga.textmode=1',
+  }
+}