path-util: Add hidden suffixes for ucf (systemd#3131)

ucf is a standard Debian helper for managing configuration file upgrades which
need more interaction or elaborate merging than conffiles managed by dpkg.
Ignore its temporary and backup files similarly to the *.dpkg-* ones to avoid
creating units for them in generators.

https://bugs.debian.org/775903

src/basic/path-util.c

@@ -774,6 +774,9 @@ bool hidden_file_allow_backup(const char *filename) {
                 endswith(filename, ".dpkg-bak") ||
                 endswith(filename, ".dpkg-backup") ||
                 endswith(filename, ".dpkg-remove") ||
+                endswith(filename, ".ucf-new") ||
+                endswith(filename, ".ucf-old") ||
+                endswith(filename, ".ucf-dist") ||
                 endswith(filename, ".swp");
 }