ibtop: Add fabric.{h,c} files

In fabric.{h,c} provides functionality for
- discovery of HCAs based on libibnetdiscover
- Querying performance counters using libmad
- Computing derived measurements (bandwidth)
ibtop: Fix bug and store bits instead of bytes

Bug fix:
As specified in the IBA specifications the port counters are
in units of (byte / 4), i.e., we need to multiply first by four
to get the actual number of bytes.

This commit also changes the code so that the number of bits
rather than bytes are stored.
ibtop: Add main() function

ibtop is a curses application for monitoring of the communication
hotspots on the HCA side. ibtop does not look at switches.
This first commit is a rudimentary version of ibtop. It displays
the rx and tx bandwidth in units of Mbps and the percentage of
the peak (ibtop is developed on an FDR cluster so the theoretical
peak is currently hardcoded).