x11/lightdm: Fix locale selection

• Allow locale selection on login screen. First, lightdm must be patched to correctly detect utf8 locales, which have a UTF-8 suffix on FreeBSD instead of utf8 like on Linux systems. Secondly, we have to use locale codes in the language menuitem as FreeBSD doesn't have support for locale naming (missing _NL_IDENTIFICATION_LANGUAGE implementation).

• Use @sample for lightdm-gtk-greeter config file to prevent it from being overwritten when the package is reinstalled / upgraded.

PR: 220614
Submitted by: pawel
MFH: 2017Q3