Support retries in certificate callback
Certificate callback doc.
Add cert callback retry test.
Compare encodings in X509_cmp as well as hash.
New function to set compression methods so they can be safely freed.
Add loaded dynamic ENGINEs to list.

Always add a dynamically loaded ENGINE to list. Otherwise it can cause
problems when multiply loaded, especially if it adds new public key methods.
For all current engines we only want a single implementation anyway.
Fix demo comment: 0.9.9 never released.
Check i before r[i].

PR#3244
(cherry picked from commit 9614d2c676ffe74ce0c919d9e5c0d622a011cbed)
typo
Clarify docs.

Remove reference to ERR_TXT_MALLOCED in the error library as that is
only used internally. Indicate that returned error data must not be
freed.
(cherry picked from commit f2d678e6e89b6508147086610e985d4e8416e867)