mesa libs issue

[Tatsuki Makino]

Correction of my mail.

> #3. Forcibly delete mesa packages.
> xargs pkg delete -f < /tmp/mesa_pkg.txt
> #4. Reinstall the required package.
> xargs portmaster < /tmp/mesa_req.txt

xargs must use with -o to run an interactive application.
ex. xargs -o pkg delete -f < /tmp/mesa_pkg.txt
It allows you to enter y.