Add check for PR_SET_NAME.

PR_SET_NAME is introduced in 2.6.9 (which is ancient, released
18 Oct 2004) but apparantly we can't count on having at least that
version.

This fixes ticket #3129.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8283 4b0bc96b-bc66-0410-9d44-ebda105a78c1