OpenVPN: New AES-GCM cipher for N2N and RW

OpenVPN: New AES-GCM cipher for N2N and RW

AES-GCM 128, 196 and 256 bit has been added to Net-to-Net and Roadwarrior section.

HMAC selection for N2N will be disabled if AES-GCM is used since GCM provides an own message authentication (GMAC).
    'auth *' line in N2N.conf will be deleted appropriately if AES-GCM is used since '--tls-auth' is not available for N2N.
HMAC selection menu for Roadwarriors is still available since '--tls-auth' is available for RWs
    which uses the configuered HMAC even AES-GCM has been applied.

Signed-off-by: Erik Kapfer <erik.kapfer@ipfire.org>