lockdown remove gnutls buffer hack

Hi,

here a patch that removes the ugly gnutls buffer hack. the gnutls read callback function should not read more than requested (specified in length) nor buffer anything for later calls. Instead it now reads as much as requested, and if it did not received the whole packet at once it re-reads repeatedly until the requested amount of data has been read. I fell over this when hunting strange errors in combination with my usbmuxd implementation.

Greetings, Nikias