Add CentOS support to /etc/system-release parser

src/_distp_esr.sh.skel

         id)
             grep -q 'Red Hat Enterprise Linux Server' $esr \
              && echo rhel
+            grep -q 'CentOS ' $esr \
+             && echo centos
             ;;
         ver)
             grep -q 'Red Hat Enterprise Linux Server release [5-6]\.[0-9]\+ ' $esr \
              && grep -o '[5-6]\.[0-9]\+' $esr
+            grep -q 'CentOS release [5-6]\.[0-9]\+ ' $esr \
+             && grep -o '[5-6]\.[0-9]\+' $esr
             ;;
         ver.x)
             _distp_esr__get ver | cut -d. -f1