Add a selftest that checks documentation invariants.

Add a selftest that checks documentation invariants.

Several approaches were discussed (mail or irc) to verify the invariants of
the GDB help documentation : checking with apropos ., modifying add_cmd
to do the check and output a warning, implement maintenance check-doc.

A selftest was finally chosen as:
  * this can be run on demand, including by users if they want
    to check user defined commands.
  * it does not interact with the normal behaviour of apropos, define,
    python, ...
    (such as output warnings when a user defines a command help that
     does not respect the doc).
  * when the selftest runs, it checks the user defined and python
    defined commands currently defined.

gdb/ChangeLog
	* unittests/help-doc-selftests.c: New file.
	* Makefile.in: Add the new file.