Backport 2.28: ssl_cache: misc improvements

[gilles-peskine-arm]

This is a backport of the bug fix in #7298: functions in the ssl_cache module were returning 1 on many error conditions, unlike the rest of the library where errors are conveyed by an error code that's always negative.

There was significant refactoring in the ssl_cache module in 3.x, so cherry-picking the commits led to significant conflicts, and instead I mostly re-did the same changes:

• Create a new error code — it had to have a different number in 2.28 because the one from 3.5 is taken in 2.28. Also, in 2.28, error.c needs to be updated.
• Change functions to return a negative error code — I searched for return 1 or ret = 1 and changed to what seemed like sensible values.
• Documentation update — I copied the text we used in 3.5.
• In 2.28, only mbedtls_ssl_cache_get and mbedtls_ssl_cache_set are concerned, there's no mbedtls_ssl_cache_remove.
• I included the changelog entry which was in a separate PR Changelog entry for 7298 #8278.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• changelog provided
• backport of ssl_cache: misc improvements #7298 + Changelog entry for 7298 #8278
• tests no, the ssl_cache module is not tested (it's used in SSL tests, but they don't really test error conditions) ← RISK!

[minosgalanakis]

LGTM