Allows for src to be kept local.

(while _pkgs is common one level above)

M  make_env.sh:
  (main): if '$H/.cpl/src/.keep_local' exists,
          don't replace it by a symlink to '$H/../src'
Take into account zip archive in ln.
Upgrade Tcl to 8.6 (from 8.5).

M .cpl/params/db
M .cpl/params/tcl
Take into account zip files from GitHub.
Use @PORT_SSHD@ in 'openssh/post_install_ssh.sh'.
Record mysql_sandbox cpanm install workaround.