tree-wide: replace strv_split_full() with strv_split_extract() everywhere

tree-wide: replace strv_split_full() with strv_split_extract() everywhere

Behaviour is not identical, as shown by the tests in test-strv.
The combination of EXTRACT_UNQUOTE without EXTRACT_RELAX only appears in
the test, so it doesn't seem particularly important. OTOH, the difference
in handling of squished parameters could make a difference. New behaviour
is what both bash and python do, so I think we can ignore this corner case.

This change has the following advantages:
- the duplication of code paths that do a very similar thing is removed
- extract_one_word() / strv_split_extract() return a proper error code.