Fix problems with squid4 to squid5 RPM update

In the squid4 to squid5 upgrade, the link /usr/share/squid/errors/es-mx becomes a directory. This change triggers a bug in rpm and yum. To work around this bug, we remove the old es-mx in the %pretrans step. There is a related problem with the /usr/share/squid/errors/es directory, whose contents cause yum (not not rpm) transaction conflict errors. It was found that changing es from a directory to a link in the %install step, and moving the old es directory to a new name in the %pretrans step, fixed the problem.