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/shared/bootspec.c

@@ -125,7 +125,7 @@ int boot_entry_load(const char *path, BootEntry *entry) {
 }
 
 void boot_config_free(BootConfig *config) {
-        unsigned i;
+        size_t i;
 
         assert(config);
 
@@ -247,7 +247,7 @@ int boot_entries_find(const char *dir, BootEntry **ret_entries, size_t *ret_n_en
 }
 
 static bool find_nonunique(BootEntry *entries, size_t n_entries, bool *arr) {
-        unsigned i, j;
+        size_t i, j;
         bool non_unique = false;
 
         assert(entries || n_entries == 0);
@@ -267,7 +267,7 @@ static bool find_nonunique(BootEntry *entries, size_t n_entries, bool *arr) {
 
 static int boot_entries_uniquify(BootEntry *entries, size_t n_entries) {
         char *s;
-        unsigned i;
+        size_t i;
         int r;
         bool arr[n_entries];