tree-wide: use c99 static for array size declarations

https://hamberg.no/erlend/posts/2013-02-18-static-array-indices.html

This only works with clang, unfortunately gcc doesn't seem to implement the check
(tested with gcc-8.2.1-5.fc29.x86_64).

Simulated error:
[2/3] Compiling C object 'systemd-nspawn@exe/src_nspawn_nspawn.c.o'.
../src/nspawn/nspawn.c:3179:45: warning: array argument is too small; contains 15 elements, callee requires at least 16 [-Warray-bounds]
                        candidate = (uid_t) siphash24(arg_machine, strlen(arg_machine), hash_key);
                                            ^                                           ~~~~~~~~
../src/basic/siphash24.h:24:64: note: callee declares array parameter as static here
uint64_t siphash24(const void *in, size_t inlen, const uint8_t k[static 16]);
                                                               ^~~~~~~~~~~~

src/core/execute.c

@@ -1595,7 +1595,7 @@ static int apply_lock_personality(const Unit* u, const ExecContext *c) {
 
 #endif
 
-static void do_idle_pipe_dance(int idle_pipe[4]) {
+static void do_idle_pipe_dance(int idle_pipe[static 4]) {
         assert(idle_pipe);
 
         idle_pipe[1] = safe_close(idle_pipe[1]);
@@ -2618,7 +2618,7 @@ out:
         return r;
 }
 
-static void append_socket_pair(int *array, size_t *n, const int pair[2]) {
+static void append_socket_pair(int *array, size_t *n, const int pair[static 2]) {
         assert(array);
         assert(n);
 
@@ -3942,7 +3942,7 @@ const char* exec_context_fdname(const ExecContext *c, int fd_index) {
         }
 }
 
-static int exec_context_named_iofds(const ExecContext *c, const ExecParameters *p, int named_iofds[3]) {
+static int exec_context_named_iofds(const ExecContext *c, const ExecParameters *p, int named_iofds[static 3]) {
         size_t i, targets;
         const char* stdio_fdname[3];
         size_t n_fds;