tree-wide: some more [static] related fixes

let's add [static] where it was missing so far

Drop [static] on parameters that can be NULL.

Add an assert() around parameters that have [static] and can't be NULL
hence.

Add some "const" where it was forgotten.

src/network/netdev/wireguard.c

@@ -899,6 +899,8 @@ static int wireguard_read_key_file(const char *filename, uint8_t dest[static WG_
         if (!filename)
                 return 0;
 
+        assert(dest);
+
         r = read_full_file_full(filename, READ_FULL_FILE_SECURE | READ_FULL_FILE_UNBASE64, &key, &key_len);
         if (r < 0)
                 return r;