924349: remove restorecon /var/cache/thumbslug

Thumbslug makes no use of /var/cache/thumbslug. I suspect this
was a copy paste error from the example SELinux packaging drafts:

http://fedoraproject.org/wiki/PackagingDrafts/SELinux

where it makes reference to /var/cache/myapp
924349: semanage changes

change semanage -a to -d in postun
redirect output of semanage to /dev/null in post

the uninstall post script was trying to ADD the thumbslug port
instead of deleting it. The -d option seems to follow the SELinux
packaging guideline for uninstall post scripts.

http://fedoraproject.org/wiki/PackagingDrafts/SELinux