Add --resume-extract option

[cperciva]

From http://mail.tarsnap.com/tarsnap-users/msg00501.html:

I should probably add a --resume-extract option which only skips files if their
metadata matches the archive.

We can't use --keep-newer-files to "resume" an extract because it will skip a file which was half-extracted. If we check the modification time and the file size we'll catch this.

[captainpete]

+1

[gperciva]

--resume-extract was added in #327, and was part of the 1.0.40 release on 2022-02-10.