[#2788] reset optarg

optarg is not reset in musl's getopt and it leaks values to other flags.
Reset it for all systems because it cannot hurt. If you remove the
optarg reset, you should see the bug in action on alpine systems in
DstubControllerTest.commandLineArgs when parsing argv2:
```
[ RUN      ] DStubControllerTest.commandLineArgs
d_controller_unittests.cc:102: Failure
Expected equality of these values:
  std::string(ex.what())
    Which is: "unsupported option: -b cfgName"
  "unsupported option: -b"
[  FAILED  ] DStubControllerTest.commandLineArgs (14 ms)
```

src/lib/process/d_controller.cc

@@ -250,6 +250,7 @@ DControllerBase::parseArgs(int argc, char* argv[]) {
     // ("c" or "d") handle it here.  If its a valid custom option, then
     // invoke customOption.
     int ch;
+    optarg = 0;
     opterr = 0;
     optind = 1;
     std::string opts("dvVWc:t:" + getCustomOpts());