Fix more mostly benign Linux/GCC compiler warnings

[droe]

In file included from src/send-internal.h:23,
                 from src/send.c:31:
src/send-linux.h:25:27: warning: ‘sockaddr’ defined but not used [-Wunused-variable]
   25 | static struct sockaddr_ll sockaddr;
      |                           ^~~~~~~~
src/send.c: In function ‘send_run’:
src/send.c:457:30: warning: ‘attempts’ may be used uninitialized [-Wmaybe-uninitialized]
  457 |         if (!zconf.dryrun && send_batch(st, batch, attempts) < 0) {
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/send.c:303:13: note: ‘attempts’ was declared here
  303 |         int attempts = zconf.retries + 1;
      |             ^~~~~~~~
src/probe_modules/module_dns.c: In function ‘dns_global_initialize’:
src/probe_modules/module_dns.c:637:25: warning: ‘__builtin_strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
  637 |                         strncpy(domain_ptr, domain_token, domain_len);
      |                         ^
src/probe_modules/module_dns.c:634:43: note: length computed here
  634 |                         uint domain_len = strlen(domain_token);
      |                                           ^~~~~~~~~~~~~~~~~~~~