Add implicit sentinel to strv_env_merge()

Just to make it a tiny bit nicer to use.

src/basic/env-util.h

@@ -39,7 +39,8 @@ char **strv_env_clean_with_callback(char **l, void (*invalid_callback)(const cha
 bool strv_env_name_is_valid(char **l);
 bool strv_env_name_or_assignment_is_valid(char **l);
 
-char **strv_env_merge(size_t n_lists, ...);
+char** _strv_env_merge(char **first, ...);
+#define strv_env_merge(first, ...) _strv_env_merge(first, __VA_ARGS__, POINTER_MAX)
 char **strv_env_delete(char **x, size_t n_lists, ...); /* New copy */
 
 char **strv_env_unset(char **l, const char *p); /* In place ... */