use basename to get_bin

When user start erlang with an absolute path, like `/usr/local/bin/erl` in yaws, erln8 can't resolve erl bin path correctly which use a relative path. The debug info is clear.
```
$  /usr/local/bin/erl
DEBUG: home directory = /home/kingbo
DEBUG: basename = erl
DEBUG: /home/kingbo/.erln8.d/otps/R17.0.1//usr/local/bin/erl
ERROR: Can't run �>n, check to see if the file exists
```