Added support for TLS Keying Material Exporters [RFC-5705]

Added support for TLS Keying Material Exporters [RFC-5705]

Keying Material Exporter [RFC-5705] allow additional keying material to be
derived from existing TLS channel. This exported keying material can then be
used for a variety of purposes.

[DS: Updated man page to document both upper and lower length boundaries]

Signed-off-by: Daniel Kubec <niel@rtfm.cz>
Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com
Acked-by: David Sommerseth <davids@redhat.com>