sysupdate: generalize feature name validity check

Let's switch to string_is_safe(), and make this available to the rest of
the sysupdate sources too.

This both relaxes and tighten the rules slightly. i.e. control character
and stuff are no longer allowed, but valid UTF-8 (as opposed to ASCII)
now is.

src/sysupdate/sysupdated.c

@@ -32,7 +32,6 @@
 #include "notify-recv.h"
 #include "os-util.h"
 #include "parse-util.h"
-#include "path-util.h"
 #include "pidref.h"
 #include "process-util.h"
 #include "runtime-scope.h"
@@ -42,7 +41,6 @@
 #include "strv.h"
 #include "sysupdate-target.h"
 #include "sysupdate-util.h"
-#include "utf8.h"
 
 #define FEATURES_DROPIN_NAME "systemd-sysupdate-enabled"
 
@@ -1404,19 +1402,6 @@ static int target_method_list_features(sd_bus_message *msg, void *userdata, sd_b
         return sd_bus_message_send(reply);
 }
 
-static bool feature_name_is_valid(const char *name) {
-        if (isempty(name))
-                return false;
-
-        if (!ascii_is_valid(name))
-                return false;
-
-        if (!filename_is_valid(strjoina(name, ".feature.d")))
-                return false;
-
-        return true;
-}
-
 static int target_method_describe_feature(sd_bus_message *msg, void *userdata, sd_bus_error *error) {
         Target *t = ASSERT_PTR(userdata);
         _cleanup_(job_freep) Job *j = NULL;
@@ -1430,7 +1415,7 @@ static int target_method_describe_feature(sd_bus_message *msg, void *userdata, s
         if (r < 0)
                 return r;
 
-        if (!feature_name_is_valid(feature))
+        if (!feature_name_valid(feature))
                 return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid feature name");
 
         if (flags != 0)
@@ -1470,7 +1455,7 @@ static int target_method_set_feature_enabled(sd_bus_message *msg, void *userdata
         r = sd_bus_message_read(msg, "sit", &feature, &enabled, &flags);
         if (r < 0)
                 return r;
-        if (!feature_name_is_valid(feature))
+        if (!feature_name_valid(feature))
                 return sd_bus_reply_method_errorf(msg,
                                                   SD_BUS_ERROR_INVALID_ARGS,
                                                   "The specified feature is invalid");