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/mempool.c

@@ -15,12 +15,12 @@
 
 struct pool {
         struct pool *next;
-        unsigned n_tiles;
-        unsigned n_used;
+        size_t n_tiles;
+        size_t n_used;
 };
 
 void* mempool_alloc_tile(struct mempool *mp) {
-        unsigned i;
+        size_t i;
 
         /* When a tile is released we add it to the list and simply
          * place the next pointer at its offset 0. */
@@ -38,8 +38,7 @@ void* mempool_alloc_tile(struct mempool *mp) {
 
         if (_unlikely_(!mp->first_pool) ||
             _unlikely_(mp->first_pool->n_used >= mp->first_pool->n_tiles)) {
-                unsigned n;
-                size_t size;
+                size_t size, n;
                 struct pool *p;
 
                 n = mp->first_pool ? mp->first_pool->n_tiles : 0;