Check if links to local files exist

[cirosantilli]

E.g. if your link does not start with https?://, it points to a local file, and so that file should exist otherwise error, e.g.:

See this code example: \a[myfile.c].

If myfile.c is renamed, we want to get an error on the cirodown build of the above line.

[cirosantilli]

OK, started implementing, and noticed that it is not in general possible without the user explicitly indicating it.

For example, https://github.com/cirosantilli/cirosantilli.github.io/tree/da296c3c933704484d5cd1a42754f60ec00f672b under https://cirosantilli.com contains links to https://cirosantilli.com/markdown-style-guide whose course lives in a non-cirodown repository.

In the server, we just can't know what random paths are mounted to what.

We should therefore just add a {local} boolean argument to those links. We can then also drop known protocol detection entirely.

This also made me notice that local links would already be breaking in case of scope rendering on subdirs with --split-headers, we can also take this chance to fix that as well.