Don't use python to check realpath of symlink

[saccarosium]

Hi, I was trying to use the application through the CLI but I was presented with this error:

  File "<string>", line 1
    import
          ^
SyntaxError: invalid syntax
/usr/local/bin/dmenu-mac: line 6: ./MacOS/dmenu-mac: No such file or directory

After looking at the script I figure to make three changes:

1. Use readlink to read the symlink path
2. Use two xargs instead of two sub-shells
3. Make it a POSIX script instead of a bash script