Locates the full path to the script bin directory to allow the use of paths
relative to the bin directory.

This allows a user to setup a directory tree for some software with
directories /bin and /lib and then the above example will allow
the use of modules in the lib directory without knowing where the software
tree is installed.

If perl is invoked using the *-e* option or the perl script is read from
'STDIN' then FindBin sets both 'Bin()' and 'RealBin()' return values to the
current directory.