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/fd-util.h

@@ -29,7 +29,7 @@ static inline int safe_close_above_stdio(int fd) {
         return safe_close(fd);
 }
 
-void close_many(const int fds[], unsigned n_fd);
+void close_many(const int fds[], size_t n_fd);
 
 int fclose_nointr(FILE *f);
 FILE* safe_fclose(FILE *f);
@@ -59,7 +59,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(DIR*, closedir);
 int fd_nonblock(int fd, bool nonblock);
 int fd_cloexec(int fd, bool cloexec);
 
-int close_all_fds(const int except[], unsigned n_except);
+int close_all_fds(const int except[], size_t n_except);
 
 int same_fd(int a, int b);