DynELF libcdb feature requrests

[TethysSvensson]

• The ability to turn downloading off and use in-memory resolving exclusively (should ignore the cache as well)
• Update in the docs to show that the libcs are cached to find the downloaded libc.
• Negative caching: If we cannot download a libc (bug, gitlab is down, the libc does not exist), this should be cached. At least an in memory negative cache, but I would prefer if we put something time-limited in /tmp as well.
• The local libc should be tried before using libcdb
• Some of the hashes are broken, e.g. https://gitlab.com/libcdb/libcdb/raw/master/hashes/build_id/ca5c6cfe528af541c3c2c15cee4b3c74da4e2fb4. Could we add a test to make sure that all the links are working?

[zachriggle]

Most of these are doable, with the exception of "Could we add a test to make sure that all the links are working?". There's no way to automate this externally, it'd have to be something done on the libcdb side.

[TethysSvensson]

Sure, I should have filed that on the libcdb repo.

[zachriggle]

Closed via #986

[TethysSvensson]

There are still a few things from the above list missing:

• The negative caching should time-limited (since libcdb might introduce more/fix broken symlinks)
• The local libc should be tried

[zachriggle]

Good catch, thanks!

[peace-maker]

The last two open items from the list are implemented now.