Add coverage-html makefile target

This uses lcov to generate a nice HTML report of test code coverage,
including branch coverage.

Signed-off-by: Kevin Murray <spam@kdmurray.id.au>
Add stanza on lcov coverage target

Adds a note about using `make coverage-html` to generate coverage
reports.

Signed-off-by: Kevin Murray <spam@kdmurray.id.au>
Tweak coverage-html target

 - Don't try to rm -rf the directory before we start: somebody might
   have set it to ~, which would be quite sad.

 - Always quote the directory name

 - Use 'make reset-gcov' before running tests.

 - Use 'make check', not ./src/test/test