Various memory leaks in configuration parsing

Various memory leaks in configuration parsing

This lot are all small issues derived from allocating new memory and
assigning to a pointer already pointing at previous allocation, or
passing xstrdup() output to a caller which does not directly hold the
passed memory.
 Both cases will disappear once we clean up the string handling in Squid
but for now these still need fixing to avoid leaking memory.

 Detected by Coverity Scan. Issues 740430, 740432, 740439.