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/escape.c

@@ -188,7 +188,7 @@ int cunescape_one(const char *p, size_t length, char32_t *ret, bool *eight_bit)
                 /* C++11 style 16bit unicode */
 
                 int a[4];
-                unsigned i;
+                size_t i;
                 uint32_t c;
 
                 if (length != (size_t) -1 && length < 5)
@@ -215,7 +215,7 @@ int cunescape_one(const char *p, size_t length, char32_t *ret, bool *eight_bit)
                 /* C++11 style 32bit unicode */
 
                 int a[8];
-                unsigned i;
+                size_t i;
                 char32_t c;
 
                 if (length != (size_t) -1 && length < 9)