Minor changes for z/OS OpenSSH V2R5

Added information on migrating to z/OS OpenSSH V2R4 or later to section Section 1.7, “Creating configuration files”.

Added note to advise against the generation of new DSA (ssh-dss) host keys in section Section 1.8, “Creating SSHD server keys”.

Updated the example sshd.sh startup script to set the _CEE_RUNOPTS and _CEE_REALLOC_CONTROL environment variables so as to improve LE memory usage in IBM z/OS V2R4 OpenSSH server.

Revised to include IBM z/OS V2R4 OpenSSH.

Revised for APAR OA54299 on IBM z/OS V2R2 OpenSSH and z/OS V2R3 OpenSSH.