- (tim) [openssh/session.c] Work around to get chroot sftp working on UnixWare
rework printing for visual host key upper border

Key types are getting longer and the current implementation of visual
host key breaks with ED25519, resulting in (note the missing bracket):

+--[ED25519  256--+

This reworks the calculation of visual host key upper border. Please be
aware that this slightly modifies the output for other key types as well:

+--[ DSA 1024]----+
+---[DSA 1024]----+

+--[ RSA 2048]----+
+---[RSA 2048]----+

+--[ECDSA  256]---+
+---[ECDSA 256]---+

+--[ED25519  256--+
+--[ED25519 256]--+