Issue 3552: Don't modify lists returned by markup list functions in interpret-markup...

Issue 3552: Don't modify lists returned by markup list functions in interpret-markup-list

This requires either
a) copying any list manually
b) interpreting markup lists right-to-left
c) more complicated and/or non-O(n) algorithms

This patch chooses b).  Modifying the lists is not really appropriate
since the lists may be created by user-written markup commands and any
problems resulting from not expecting modifications of the return
value might be quite hard to debug.