util: Format negative zero as '-0'

Format negative zero as '-0' instead of as '0', as it does not behave
identically to positive zero. ((-0).toString() still returns '0' as
required by ES5 9.8.1.2).

Fixes joyent/node#6548.
Closes joyent/node#6550.
tls: add serialNumber to getPeerCertificate()

Add a 'serialNumber' property to the object that is returned by
tls.CryptoStream#getPeerCertificate().  Contains the certificate's
serial number encoded as a hex string.  The format is identical to
`openssl x509 -serial -in path/to/certificate`.

Fixes #6583.