Config parser doesn't accept multiple IdentityFile lines

[Tilo Eckert]

Hi,

~/.ssh/config can contain multiple lines with the "IdentityFile" keyword 
to add multiple keys to try in public key authentication.

The libssh config parser ignores all but the first IdentityFile line, 
resulting in failed authentication if the key that is accepted by the 
server is not the first.

The issue seems to be the "seen" array in ssh_config_parse_line() which 
already has Host, Match and Include as exceptions. Probably just need to 
add IdentityFile.

Regards
Tilo