Code coverage report

[Simon Josefsson]

All,

In git master you can now generate code coverage reports using 'make
coverage'.  You need the lcov tools.  Output is also available from:

http://www.gnu.org/software/gnutls/coverage/

The lib/* sub-directories are probably the most interesting.

It would be great if someone wants to contribute new self-tests that
exercises code paths that aren't tested right now.  Today the self-tests
invoke around 48.9% of the library, which could be improved.

/Simon