Merge branch 'sj/string-list'

Code and test clean-up around string-list API.

* sj/string-list:
  u-string-list: move "remove duplicates" test to "u-string-list.c"
  u-string-list: move "filter string" test to "u-string-list.c"
  u-string-list: move "test_split_in_place" to "u-string-list.c"
  u-string-list: move "test_split" into "u-string-list.c"
  string-list: enable sign compare warnings check
  string-list: return index directly when inserting an existing element
  string-list: remove unused "insert_at" parameter from add_entry
  string-list: fix sign compare warnings for loop iterator

diff --cc Makefile
Simple merge

diff --cc t/meson.build
Simple merge