Move Bitmask struct back to vpn package

pkg/backend/status.go

pkg/bitmask/init.go

pkg/bitmask/interface.go

pkg/bitmask/motd_test.go

pkg/bitmask/testdata/broken.pem

pkg/bitmask/testdata/empty.pem

pkg/bitmask/testdata/expired.pem

pkg/bitmask/testdata/privatekeymissing.pem

pkg/bitmask/utils_test.go

pkg/vpn/bitmask.go

pkg/vpn/interface.go

pkg/vpn/motd_test.go

pkg/vpn/openvpn.go

pkg/vpn/remote_selection.go

pkg/vpn/status.go

pkg/vpn/testdata/broken.pem

pkg/vpn/testdata/empty.pem

pkg/vpn/testdata/expired.pem

pkg/vpn/testdata/privatekeymissing.pem

pkg/vpn/utils.go