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/time-util.c

@@ -434,7 +434,7 @@ char *format_timespan(char *buf, size_t l, usec_t t, usec_t accuracy) {
                 { "us",    1               },
         };
 
-        unsigned i;
+        size_t i;
         char *p = buf;
         bool something = false;
 
@@ -612,7 +612,7 @@ static int parse_timestamp_impl(const char *t, usec_t *usec, bool with_tz) {
         time_t x;
         usec_t x_usec, plus = 0, minus = 0, ret;
         int r, weekday = -1, dst = -1;
-        unsigned i;
+        size_t i;
 
         /*
          * Allowed syntaxes:
@@ -960,7 +960,7 @@ static char* extract_multiplier(char *p, usec_t *multiplier) {
                 { "us",      1ULL            },
                 { "µs",      1ULL            },
         };
-        unsigned i;
+        size_t i;
 
         for (i = 0; i < ELEMENTSOF(table); i++) {
                 char *e;
@@ -1134,8 +1134,8 @@ int parse_nsec(const char *t, nsec_t *nsec) {
 
         for (;;) {
                 long long l, z = 0;
+                size_t n = 0, i;
                 char *e;
-                unsigned i, n = 0;
 
                 p += strspn(p, WHITESPACE);