Correct permissions for new AEA files

When extracted, the AEA folder containing the LaTeX files does
not have correct permissions and thus cannot be read by mktexlsr.
The permissions on all folders are now set by install-tl-ubuntu.
Overwrite the /opt/texbin symlink if it exists

This arises if there was a previous TeX Live installation.
For example, if installing TeX Live 2014 when Tex Live 2013
was previously installed and not manually removed.

A warning is given to the user in case he/she would like to
manually remove the old TeX Live installation.