sshd: don't use argv[0] as PAM service name

sshd: don't use argv[0] as PAM service name

sshd would implicitly use argv[0] as the PAM service name to
allow people to select different PAM service names by making
differently-named copies/links to the sshd binary.

Splitting sshd into sshd/sshd-session broke this, as the process
that starts PAM is always sshd-session and the user has no control
over this.

Hardcode "sshd" as the default PAM service name unless/until we
figure out a better way. Should unbreak OSX integration tests.