tree-wide: be more careful with the type of array sizes

Previously we were a bit sloppy with the index and size types of arrays,
we'd regularly use unsigned. While I don't think this ever resulted in
real issues I think we should be more careful there and follow a
stricter regime: unless there's a strong reason not to use size_t for
array sizes and indexes, size_t it should be. Any allocations we do
ultimately will use size_t anyway, and converting forth and back between
unsigned and size_t will always be a source of problems.

Note that on 32bit machines "unsigned" and "size_t" are equivalent, and
on 64bit machines our arrays shouldn't grow that large anyway, and if
they do we have a problem, however that kind of overly large allocation
we have protections for usually, but for overflows we do not have that
so much, hence let's add it.

So yeah, it's a story of the current code being already "good enough",
but I think some extra type hygiene is better.

This patch tries to be comprehensive, but it probably isn't and I missed
a few cases. But I guess we can cover that later as we notice it. Among
smaller fixes, this changes:

1. strv_length()' return type becomes size_t

2. the unit file changes array size becomes size_t

3. DNS answer and query array sizes become size_t

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=76745

src/basic/string-util.h

@@ -109,7 +109,7 @@ char *strjoin_real(const char *x, ...) _sentinel_;
                 const char *_appendees_[] = { a, __VA_ARGS__ };         \
                 char *_d_, *_p_;                                        \
                 size_t _len_ = 0;                                          \
-                unsigned _i_;                                           \
+                size_t _i_;                                           \
                 for (_i_ = 0; _i_ < ELEMENTSOF(_appendees_) && _appendees_[_i_]; _i_++) \
                         _len_ += strlen(_appendees_[_i_]);              \
                 _p_ = _d_ = alloca(_len_ + 1);                          \