unit: instead of directly loading drop-in configuration snippets use conf_files_list_strv()

This has the benefit of allowing the usual overriding/masking knowledge
everybody loves so much.

src/shared/strv.c

@@ -370,6 +370,21 @@ fail:
         return NULL;
 }
 
+int strv_extend(char ***l, const char *value) {
+        char **c;
+
+        if (!value)
+                return 0;
+
+        c = strv_append(*l, value);
+        if (!c)
+                return -ENOMEM;
+
+        strv_free(*l);
+        *l = c;
+        return 0;
+}
+
 char **strv_uniq(char **l) {
         char **i;
 

src/shared/strv.h

@@ -37,6 +37,7 @@ unsigned strv_length(char **l);
 char **strv_merge(char **a, char **b);
 char **strv_merge_concat(char **a, char **b, const char *suffix);
 char **strv_append(char **l, const char *s);
+int strv_extend(char ***l, const char *value);
 
 char **strv_remove(char **l, const char *s);
 char **strv_remove_prefix(char **l, const char *s);