strv: rework FOREACH_STRING() macro

strv: rework FOREACH_STRING() macro

So it's apparently problematic that we use STRV_MAKE() (i.e. a compound
initializer) outside of the {} block we use it in (and that includes
outside of the ({}) block, too). Hence, let's rework the macro to not
need that.

This also makes the macro shorter, which is definitely a good and more
readable. Moreover, it will now complain if the iterator is a "char*"
instead of a "const char*", which is good too.

Fixes: #11394