Implement KEYMGMT in the xkey provider

Implement KEYMGMT in the xkey provider

A minimal set of functions for keymgmt are implemented.
No support for external key import as yet, only native
keys. Support for native keys is required as keys may
get imported into us for some operations as well as
for comparison with unexportable external keys that we hold.

Implementation of signature callbacks is in the next commit.

v2 changes: This was commit 3/9 in v1
v3 changes: When OpenSSL native key is imported instead of duplicating
the whole key, use only the public components for public key.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-3-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23438.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>