Improve adf_salv (salvage)

[t-w]

The adf_salv module, with the very useful undel functions, needs improvements in terms of:

• memory management (ie. memory leaks)
• error checking (eg. undeleting things not checking if the blocks are read correctly)

(Since in some cases possibly the error check should be avoided (even if a checksum is not correct, data could be recovered), the issue is created to take notes/comments on this).

[lclevy]

Inspiration was disksalv https://www.nightvzn.net/portfolio/web/amiga_monitor/archives/1_5html/disksalv.htm

[t-w]

Good to know!

[t-w]

There is still (at least...) one thing left:

• adfUndelFile(): add reverting adfCreateEntry in case adfAddInCache fails. While this should be relatively rare case, nevertheless, it should be fixed/improved.