Use "system" for "use system language", and don't try to print a null string.

Use "system" for "use system language", and don't try to print a null string.

Add a #define USE_SYSTEM_LANGUAGE for the language string meaning "use
the system setting", and use that instead of hardcoding "system" in
various places.

If "language" is null, don't try to write it to the file with fprintf()
- on *most* systems, that prints "(null)", but on some systems, such as
Solaris, it *crashes*.  Write USE_SYSTEM_LANGUAGE instead.  Check for
"(null)" and treat it as meaning "use the system language".

Map "auto" to "use the system language" as well, for backwards
compatibility.

Change-Id: Iba9be540a5139e9cca8bddd0761ee4cbf0f79a49
Reviewed-on: https://code.wireshark.org/review/15147
Reviewed-by: Guy Harris <guy@alum.mit.edu>