tree-wide: unify how we pick OS pretty name to display

src/basic/os-util.c

@@ -370,3 +370,12 @@ int os_release_support_ended(const char *support_end, bool quiet) {
         usec_t ts = now(CLOCK_REALTIME);
         return DIV_ROUND_UP(ts, USEC_PER_SEC) > (usec_t) eol;
 }
+
+const char *os_release_pretty_name(const char *pretty_name, const char *name) {
+        /* Distills a "pretty" name to show from os-release data. First argument is supposed to be the
+         * PRETTY_NAME= field, the second one the NAME= field. This function is trivial, of course, and
+         * exists mostly to ensure we use the same logic wherever possible. */
+
+        return empty_to_null(pretty_name) ?:
+                empty_to_null(name) ?: "Linux";
+}

src/basic/os-util.h

@@ -33,3 +33,5 @@ int load_os_release_pairs(const char *root, char ***ret);
 int load_os_release_pairs_with_prefix(const char *root, const char *prefix, char ***ret);
 
 int os_release_support_ended(const char *support_end, bool quiet);
+
+const char *os_release_pretty_name(const char *pretty_name, const char *name);