luci-base(-libs): move pcdata() and striptags() from util- to xml-class

To complete the previous commit these functions are defined in the resulting
luci-base package but are also used in the new luci-base-libs package. So
move them into the new xml-module of the new package.

Signed-off-by: Sven Roederer <freifunk@it-solutions.geroedel.de>