publicKey data can contain NUL, base64 encode at source

publicKey data can contain NUL, base64 encode at source

One *mustn't* treat the publicKey as common string, it it's
effectively binary data, and thus can contain 0 at any position.

If we pass it to perl via setpv (PV being string), things may get
cutoff and a wrong pubKey may get saved, thus locking an user out as
it cannot get verified anymore!

Use a simple & fast base64 encoding immediately at the source, using
the correct keylength (U2FS_PUBLIC_KEY_LEN) we can use that.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>