symbol string_list_appendf() unused

Hi Martin,

I strongly suspect that I haven't followed the discussion on
the list closely enough, but your 'ma/unpack-trees-free-msgs'
branch in 'pu', seems to define string_list_appendf() but then
never call it. This is despite commit 40ebd6c7b0 ("string-list:
provide `string_list_appendf()`", 2018-05-20) claiming that:
'The next commit will add a user'. ;-)

Have I missed something?

ATB,
Ramsay Jones