Coverage support: build with --enable-coverage to have tests run with gcov

If you pass the --enable-coverage flag on the command line, we build
our testing binaries with appropriate options eo enable coverage
testing.  We also build a "tor-cov" binary that has coverage enabled,
for integration tests.

On recent OSX versions, test coverage only works with clang, not gcc.
So we warn about that.

Also add a contrib/coverage script to actually run gcov with the
appropriate options to generate useful .gcov files.  (Thanks to
automake, the .o files will not have the names that gcov expects to
find.)

Also, remove generated gcda and gcno files on clean.