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/strv.h

@@ -32,7 +32,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(char**, strv_free_erase);
 void strv_clear(char **l);
 
 char **strv_copy(char * const *l);
-unsigned strv_length(char * const *l) _pure_;
+size_t strv_length(char * const *l) _pure_;
 
 int strv_extend_strv(char ***a, char **b, bool filter_duplicates);
 int strv_extend_strv_concat(char ***a, char **b, const char *suffix);
@@ -41,7 +41,7 @@ int strv_extendf(char ***l, const char *format, ...) _printf_(2,0);
 int strv_extend_front(char ***l, const char *value);
 int strv_push(char ***l, char *value);
 int strv_push_pair(char ***l, char *a, char *b);
-int strv_insert(char ***l, unsigned position, char *value);
+int strv_insert(char ***l, size_t position, char *value);
 
 static inline int strv_push_prepend(char ***l, char *value) {
         return strv_insert(l, 0, value);
@@ -113,7 +113,7 @@ void strv_print(char **l);
                 if (!first)                                     \
                         _l = (char**) &first;                   \
                 else {                                          \
-                        unsigned _n;                            \
+                        size_t _n;                              \
                         va_list _ap;                            \
                                                                 \
                         _n = 1;                                 \