Use paths specified from environment variables for /etc configuration files

Some configuration files that need updates are directly under in /etc. To
update them atomically, we need write access to /etc. For Ubuntu Core this is
an issue as /etc is not writable. Only a selection of subdirectories can be
writable. The general solution is symlinks or bind mounts to writable places.
But for atomic writes in /etc, that does not work. So Ubuntu has had a patch
for that that did not age well.

Instead we would like to introduce some environment variables for alternate
paths.

 * SYSTEMD_ETC_HOSTNAME: /etc/hostname
 * SYSTEMD_ETC_MACHINE_INFO: /etc/machine-info
 * SYSTEMD_ETC_LOCALTIME: /etc/localtime
 * SYSTEMD_ETC_LOCALE_CONF: /etc/locale.conf
 * SYSTEMD_ETC_VCONSOLE_CONF: /etc/vconsole.conf
 * SYSTEMD_ETC_ADJTIME: /etc/adjtime

While it is for now expected that there is a symlink from the standard, we
still try to read them from that alternate path. This is important for
`/etc/localtime`, which is a symlink, so we cannot have an indirect symlink or
bind mount for it.

Since machine-id is typically written only once and not updated. This commit
does not cover it. An initrd can properly create it and bind mount it.

src/basic/time-util.c

@@ -1607,7 +1607,7 @@ int get_timezone(char **ret) {
 
         assert(ret);
 
-        r = readlink_malloc("/etc/localtime", &t);
+        r = readlink_malloc(etc_localtime(), &t);
         if (r == -ENOENT)
                 /* If the symlink does not exist, assume "UTC", like glibc does */
                 return strdup_to(ret, "UTC");
@@ -1623,6 +1623,15 @@ int get_timezone(char **ret) {
         return strdup_to(ret, e);
 }
 
+const char* etc_localtime(void) {
+        static const char *cached = NULL;
+
+        if (!cached)
+                cached = secure_getenv("SYSTEMD_ETC_LOCALTIME") ?: "/etc/localtime";
+
+        return cached;
+}
+
 int mktime_or_timegm_usec(
                 struct tm *tm, /* input + normalized output */
                 bool utc,