Running picoquicdemo with absolute path

Some users have experienced issues when running the "picoquicdemo" program from another location than the source directory, see for example issue 1431. This is because, in some conditions, the program looks for data files inside the source directory.

When running as a server, the demo program looks for the "key" parameter and the "cert" parameter. If they are not set explicitly, it looks for the default test versions in the "certs" folder of the distribution. The simple mitigation is to provide the path to these key and cert files using the command line arguments "-k" and "-c".

We could modify the demo program and add support for a "location of the source" argument similar to what is available in the test program, but the case is weak. The test suites uses a large number of input or reference files for test, and passing all of them on the command line would be really cumbersome. The demo program only uses two files, and in a normal production environment admins are supposed to provide their own key and cert...