Fix two warnings found by gcc-9.2:

Fix two warnings found by gcc-9.2:

sockssrv.c: In function 'main':
sockssrv.c:430:17: warning: declaration of 'c' shadows a previous local [-Wshadow]
  430 |   struct client c;
      |                 ^
sockssrv.c:379:6: note: shadowed declaration is here
  379 |  int c;
      |      ^
sockssrv.c:405:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
  405 |     dprintf(2, "error: option -%c requires an operand\n", optopt);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sockssrv.c:406:4: note: here
  406 |    case '?':
      |    ^~~~