Support ksh versions

Korn shell had two major versions: ’88 and ’93. Some systems have ksh
installed under the name `ksh93`. A few systems (maybe only Solaris
now) also have a `ksh88`. A few others use the `pdksh` (roughly ’88)
or `mksh` (roughly ’93) implementations, originated before ksh was
open source.

As far as the (very minor) use in rbenv is concerned, these are all
equivalent. This change accepts all of the above.

Tested with `ksh93`, `pdksh`, and `mksh`.

libexec/rbenv-init

@@ -62,7 +62,7 @@ if [ -z "$print" ]; then
   zsh )
     profile='~/.zshrc'
     ;;
-  ksh )
+  ksh | ksh93 | ksh88 | pdksh | mksh )
     profile='~/.profile'
     ;;
   fish )
@@ -144,7 +144,7 @@ function rbenv
 end
 EOS
   ;;
-ksh )
+ksh | ksh93 | ksh88 | pdksh | mksh )
   cat <<EOS
 function rbenv {
   typeset command