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.c

@@ -107,8 +107,8 @@ char **strv_copy(char * const *l) {
         return r;
 }
 
-unsigned strv_length(char * const *l) {
-        unsigned n = 0;
+size_t strv_length(char * const *l) {
+        size_t n = 0;
 
         if (!l)
                 return 0;
@@ -122,7 +122,7 @@ unsigned strv_length(char * const *l) {
 char **strv_new_ap(const char *x, va_list ap) {
         const char *s;
         char **a;
-        unsigned n = 0, i = 0;
+        size_t n = 0, i = 0;
         va_list aq;
 
         /* As a special trick we ignore all listed strings that equal
@@ -257,7 +257,7 @@ int strv_extend_strv_concat(char ***a, char **b, const char *suffix) {
 char **strv_split(const char *s, const char *separator) {
         const char *word, *state;
         size_t l;
-        unsigned n, i;
+        size_t n, i;
         char **r;
 
         assert(s);
@@ -287,7 +287,7 @@ char **strv_split(const char *s, const char *separator) {
 
 char **strv_split_newlines(const char *s) {
         char **l;
-        unsigned n;
+        size_t n;
 
         assert(s);
 
@@ -380,7 +380,7 @@ char *strv_join(char **l, const char *separator) {
 
 int strv_push(char ***l, char *value) {
         char **c;
-        unsigned n, m;
+        size_t n, m;
 
         if (!value)
                 return 0;
@@ -405,7 +405,7 @@ int strv_push(char ***l, char *value) {
 
 int strv_push_pair(char ***l, char *a, char *b) {
         char **c;
-        unsigned n, m;
+        size_t n, m;
 
         if (!a && !b)
                 return 0;
@@ -431,9 +431,9 @@ int strv_push_pair(char ***l, char *a, char *b) {
         return 0;
 }
 
-int strv_insert(char ***l, unsigned position, char *value) {
+int strv_insert(char ***l, size_t position, char *value) {
         char **c;
-        unsigned n, m, i;
+        size_t n, m, i;
 
         if (!value)
                 return 0;
@@ -601,7 +601,7 @@ char **strv_parse_nulstr(const char *s, size_t l) {
          */
 
         const char *p;
-        unsigned c = 0, i = 0;
+        size_t c = 0, i = 0;
         char **v;
 
         assert(s || l <= 0);
@@ -765,7 +765,7 @@ int strv_extendf(char ***l, const char *format, ...) {
 }
 
 char **strv_reverse(char **l) {
-        unsigned n, i;
+        size_t n, i;
 
         n = strv_length(l);
         if (n <= 1)