`hh_server --check` use the right error code

Summary: This has annoyed a lot of folks, let's fix. We `exit 0` on no
errors, `exit 1` on errors.

Reviewed By: @fredemmott

Differential Revision: D1403902
Fix reuse of dead persistent sockets

Summary: This fixes a bug where dead persistent sockets could be re-used
because the return value of the recv() syscall wasn't properly
checked. Previously, if recv returned -1 to signal an error
without setting errno to EAGAIN, the socket was erroneously
reused. This brings the persistent socket reuse check to
parity with Zend.

Also removed an outdated comment.
Closes #3004

Reviewed By: @ptarjan

Differential Revision: D1398661

Pulled By: @JoelMarcey