Fix 'make check'

Fix 'make check'

Add <string.h> where needed to fix compilation of libgetopt++ tests

UserSetting was removed in f26f525f, so remove that test.

UserSettings also got rewritten in that commit, so rewrite test.

Rework UserSettings constructor so it can be initialized without needing
to attempt loading from a file (which requires all the io_stream URL
providers we might try to be registered).

Run 'make check' in appveyor.