set pid_=-1 after wait, actually implement pid()

Summary: minor issues in folly::Subprocess

Test Plan: subprocess_test

Reviewed By: philipp@fb.com

FB internal diff: D680959

folly/Subprocess.cpp

@@ -427,6 +427,7 @@ ProcessReturnCode Subprocess::wait() {
   checkUnixError(found, "waitpid");
   DCHECK_EQ(found, pid_);
   returnCode_ = ProcessReturnCode(status);
+  pid_ = -1;
   return returnCode_;
 }
 
@@ -441,6 +442,10 @@ void Subprocess::sendSignal(int signal) {
   checkUnixError(r, "kill");
 }
 
+pid_t Subprocess::pid() const {
+  return pid_;
+}
+
 namespace {
 
 std::pair<const uint8_t*, size_t> queueFront(const IOBufQueue& queue) {