Make NIC RX/TX buffer sizes configureable in libc

* Remove far too low default values from Nic::Connection constructor
* Extend lwip initialization function with desired TX/RX buffer sizes
* Add configuration possibility to libc_lwip_dhcp plugin to define
  buffer sizes, like the following:

  '<libc tx_buf_size="1M" tx_buf_size="1M"/>'

Fixes #892