dpkg: upgrade 1.21.17 -> 1.21.18

dpkg: upgrade 1.21.17 -> 1.21.18

Changelog:
==========
 * Perl modules:
    - Dpkg::BuildFlags: Fix strip method to always work with duplicates.
  * Documentation:
    - man: Add a note to dpkg-fsys-usrunmess(8) about Debian support.
  * Code internals:
    - libdpkg: Move compression level max bound check from dpkg-deb.
    - libdpkg: Switch dpkg_lzma_strerror() to use struct io_lzma.
    - libdpkg: Split compression filter operation tracking from its status.
    - libdpkg: Generalize compression stream action handling.
    - libdpkg: Abstract compression stream status tracking.
    - libdpkg: Add zstd support for .deb archives. Closes: #892664
  * Build system:
    - Update .mailmap mappings.
  * Packaging:
    - Update copyright years.
  * Test suite:
    - Add dpkg-deb build and extract tests.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>