Add key server preferences signature subpacket parsing to PGPPacket

Add key server preferences signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (N octets of flags)

  This is a list of one-bit flags that indicate preferences that the
  key holder has about how the key is handled on a key server.  All
  undefined flags MUST be zero.

  First octet: 0x80 = No-modify
    the key holder requests that this key only be modified or updated
    by the key holder or an administrator of the key server.

  This is found only on a self-signature.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.17