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/io-util.h

@@ -28,9 +28,8 @@ int fd_wait_for_event(int fd, int event, usec_t timeout);
 
 ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length);
 
-static inline size_t IOVEC_TOTAL_SIZE(const struct iovec *i, unsigned n) {
-        unsigned j;
-        size_t r = 0;
+static inline size_t IOVEC_TOTAL_SIZE(const struct iovec *i, size_t n) {
+        size_t j, r = 0;
 
         for (j = 0; j < n; j++)
                 r += i[j].iov_len;
@@ -38,8 +37,8 @@ static inline size_t IOVEC_TOTAL_SIZE(const struct iovec *i, unsigned n) {
         return r;
 }
 
-static inline size_t IOVEC_INCREMENT(struct iovec *i, unsigned n, size_t k) {
-        unsigned j;
+static inline size_t IOVEC_INCREMENT(struct iovec *i, size_t n, size_t k) {
+        size_t j;
 
         for (j = 0; j < n; j++) {
                 size_t sub;