locale-util: add new helper locale_is_installed()

This new helper checks whether the specified locale is installed. It's
distinct from locale_is_valid() which just superficially checks if a
string looks like something that could be a valid locale.

Heavily inspired by @jsynacek's #13964.

Replaces: #13964

src/basic/locale-util.h

@@ -31,6 +31,7 @@ typedef enum LocaleVariable {
 
 int get_locales(char ***l);
 bool locale_is_valid(const char *name);
+int locale_is_installed(const char *name);
 
 #define _(String) gettext(String)
 #define N_(String) String