bsdtar: Support `--mtime` and `--clamp-mtime` (#2601)

bsdtar: Support `--mtime` and `--clamp-mtime` (#2601)

Hi,

This PR adds support for setting a forced mtime on all written files
(`--mtime` and `--clamp-mtime`) in bsdtar.

The end goal will be to support all functionalities in
<https://reproducible-builds.org/docs/archives/#full-example>, namely
`--sort` and disabling other attributes (atime, ctime, etc.).

Fixes #971.

## History

- [v1](https://github.com/zhaofengli/libarchive/tree/forced-mtime-v1):
Added `archive_read_disk_set_forced_mtime` in libarchive. As a result,
it was only applied when reading from the filesystem and not from other
archives.
- [v2](https://github.com/zhaofengli/libarchive/tree/forced-mtime-v2):
Refactored to apply the forced mtime in `archive_write`.
- v3 (current): Reduced libarchive change to exposing
`archive_parse_date`, moved clamping logic into bsdtar.

---------

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>
Co-authored-by: Dustin L. Howett <dustin@howett.net>