tmpfiles: optionally, decode string to write to files with base64

This is useful to use "f" or "w" to write arbitrary binary files to
disk, or files with newlines and similar (for example to provision SSH
host keys and similar).

test/test-systemd-tmpfiles.py

@@ -188,6 +188,14 @@ def test_hard_cleanup(*, user):
     label = 'valid_symlink-deep'
     test_content('f= {} - - - - ' + label, label, user=user, subpath='/deep/1/2', path_cb=valid_symlink)
 
+def test_base64():
+    test_line('f~ /tmp/base64-test - - - - UGlmZgpQYWZmClB1ZmYgCg==', user=False, returncode=0)
+
+    with open("/tmp/base64-test", mode='r') as f:
+        d = f.read()
+
+    assert d == "Piff\nPaff\nPuff \n"
+
 if __name__ == '__main__':
     test_invalids(user=False)
     test_invalids(user=True)
@@ -198,3 +206,5 @@ if __name__ == '__main__':
 
     test_hard_cleanup(user=False)
     test_hard_cleanup(user=True)
+
+    test_base64()