Install the symlinks unconditionally. This way the user don't need
to reinstall completion after adding an extension. Of course fine-
grained control is possible with: `make install EXTENSIONS=web` e.g.

Most of the included files are meant to be output literally, without any
macro processing. Using `undivert` in place of `include` achieves that.
This is a safety net against unsanitized input generated during the
build. Also, developers editing the shell code stub shouldn't be
constantly alert about triggering accidental macro expansion.
In order that `undivert` mimics GNU behaviour on BSDs, the `-g` flag is
used for the m4 invocation.

Just print a reminder to use `gmake` instead of spurting a bunch
of errors.

Better be safe that sorry...

* `help` command's output is no longer listing help topics, so
those completions are removed.
* `check` command supersedes `check-dates`, `check-dupes`, etc.