core: Execute first boot presets in an enable-only preset-mode.

This means any existing enabled units well be preserved and no
pre-created symlinks will be removed. This is done on first boot, when
the assumption is that /etc is not populated at all (no machine-id
setup). For minimal containers that gives a significant first boot
speed up, approximately ~20ms / ~16% in my trials.

TODO

@@ -149,10 +149,6 @@ Features:
   that are not supported...
   http://lists.freedesktop.org/archives/systemd-devel/2015-February/028076.html
 
-* PID 1: when invoking systemctl preset-all on first boots, operate in
-  an exclusively additive way, i.e. never remove any pre-existing
-  symlinks, only add new ones.
-
 * Introduce $LISTEN_NAMES to complement $LISTEN_FDS, containing a
   colon separated list of identifiers for the fds passed.
 

src/core/main.c

@@ -1618,7 +1618,7 @@ int main(int argc, char *argv[]) {
                 bump_rlimit_nofile(&saved_rlimit_nofile);
 
                 if (empty_etc) {
-                        r = unit_file_preset_all(UNIT_FILE_SYSTEM, false, NULL, UNIT_FILE_PRESET_FULL, false, NULL, 0);
+                        r = unit_file_preset_all(UNIT_FILE_SYSTEM, false, NULL, UNIT_FILE_PRESET_ENABLE_ONLY, false, NULL, 0);
                         if (r < 0)
                                 log_warning_errno(r, "Failed to populate /etc with preset unit settings, ignoring: %m");
                         else