tree-wide: make specifier expansion --root= aware

This fixes repart's, systemctl's, sysusers' and tmpfiles' specifier
expansion to honour the root dir specified with --root=. This is
relevant for specifiers such as %m, %o, … which are directly sourced
from files on disk.

This doesn't try to be overly smart: specifiers referring to runtime
concepts (i.e. boot ID, architecture, hostname) rather than files on the
medium are left as is. There's certainly a point to be made that they
should fail in case --root= is specified, but I am not entirely convinced
about that, and it's certainly something we can look into later if
there's reason to.

I wondered for a while how to hook this up best, but given that quite a
large number of specifiers resolve to data from files on disks, and most
of our tools needs this, I ultimately decided to make the root dir a
first class parameter to specifier_printf().

Replaces: #16187
Fixes: #16183

src/resolve/resolved-conf.c

@@ -255,7 +255,7 @@ int config_parse_dnssd_service_name(
                 return 0;
         }
 
-        r = specifier_printf(rvalue, DNS_LABEL_MAX, specifier_table, NULL, &name);
+        r = specifier_printf(rvalue, DNS_LABEL_MAX, specifier_table, NULL, NULL, &name);
         if (r < 0) {
                 log_syntax(unit, LOG_WARNING, filename, line, r,
                            "Invalid service instance name template '%s', ignoring assignment: %m", rvalue);

src/resolve/resolved-dnssd.c

@@ -135,7 +135,7 @@ static int dnssd_service_load(Manager *manager, const char *filename) {
         return 0;
 }
 
-static int specifier_dnssd_host_name(char specifier, const void *data, const void *userdata, char **ret) {
+static int specifier_dnssd_host_name(char specifier, const void *data, const char *root, const void *userdata, char **ret) {
         DnssdService *s  = (DnssdService *) userdata;
         char *n;
 
@@ -170,7 +170,7 @@ int dnssd_render_instance_name(DnssdService *s, char **ret_name) {
         assert(s);
         assert(s->name_template);
 
-        r = specifier_printf(s->name_template, DNS_LABEL_MAX, specifier_table, s, &name);
+        r = specifier_printf(s->name_template, DNS_LABEL_MAX, specifier_table, NULL, s, &name);
         if (r < 0)
                 return log_debug_errno(r, "Failed to replace specifiers: %m");