asdf.sh: support mksh

The mksh shell (and perhaps others too) will not be able to determine a
sourced script's path by looking at either `${BASH_SOURCE[0]}` or `$0`.

By capturing the value of `$_` as the very first thing the script does,
`asdf.sh` can use that as the input for `dirname` in order to determine
a value to use for `$ASDF_DIR`.

I have tested this change with mksh to confirm that it works, and have
also tested it with both bash and zsh to verify that neither of those
stop working.

This resolves the only compatibility issue I've ran into with asdf,
caused by mksh sourcing a script. All subsequent asdf usage involves an
asdf executable doing the sourcing, so it's bash to bash at that point.

asdf.sh

@@ -1,6 +1,9 @@
 #!/usr/bin/env bash
 
-if [ "${BASH_SOURCE[0]}" != "" ]; then
+_under="$_"
+if [[ "$_under" == *".sh" ]]; then
+  current_script_path="$_under"
+elif [ "${BASH_SOURCE[0]}" != "" ]; then
   current_script_path="${BASH_SOURCE[0]}"
 else
   current_script_path="$0"