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/hostname-util.h

@@ -37,6 +37,9 @@ static inline bool is_dns_proxy_stub_hostname(const char *hostname) {
         return STRCASE_IN_SET(hostname, "_localdnsproxy", "_localdnsproxy.");
 }
 
+const char* etc_hostname(void);
+const char* etc_machine_info(void);
+
 int get_pretty_hostname(char **ret);
 
 int machine_spec_valid(const char *s);