Remove use of PACKAGE_STRING

PACKAGE_VERSION is more explicit, and also, we don't pretend that changing the
project name in meson.build has any real effect. "systemd" is embedded in a
thousand different places, so let's just use the hardcoded string consistently.
This is mostly in preparation for future changes.

src/basic/util.c

@@ -557,7 +557,7 @@ uint64_t system_tasks_max_scale(uint64_t v, uint64_t max) {
 }
 
 int version(void) {
-        puts(PACKAGE_STRING "\n"
+        puts("systemd " PACKAGE_VERSION "\n"
              SYSTEMD_FEATURES);
         return 0;
 }