Too wide function clause was used in ssl_connection which led to ssl
connection process crashes when `{hibernate_after, N}` with extremely
small N was passed among other options to `ssl:connect`.

TLS hash_sign algorithms may have proprietary values see
http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml

We should add callbacks to let applications handle them.
But for now we do not want to crash if they are present and
let other algorithms be negotiated.

alert records needs to be thrown from
ssl_handshake:premaster_secret/[2/3] so that operations will end up in
the catch clause of the invokation of certify_client_key_exchange/3 in
ssl_connection.erl, and hence terminate gracefully and not continue to try
and calculate the master secret with invalid inputs and crash.

Correct mistake

For comparison with file time stamps os:timestamp makes more sense
and is present in 17 as well as 18.

The PEM cache is now validated by a background process, instead of
always keeping it if it is small enough and clearing it otherwhiss.
That strategy required that small caches where cleared by API function
if a file changes on disk.

However document the clearing API function as it can still be usefull.

A selfsigned trusted anchor should not be in the certifcate chain passed to
the certificate path validation.

Conflicts:
	lib/ssl/src/ssl_certificate.erl