Convert all user-definable defvar into defcustom in bison-mode defgroup.

[pierre-rouleau]

• Add the defgroup bison-mode, under the defgroup for C.
• All defcustom are in this new defgroup.
• This has no impact on users of Emacs 20 and later when defgroup and defcustom were introduced (in 1997). Since the bison-mode.el was created in 1998, the probability that some users may be affected exists but is very low.
• Users can continue to set the variables as if they were declared as defvars. They can now use Emacs customization interface to control the values of the user-definable variables.
• Minor cleanup for the docstrings of the affected forms.
• Minor trailing whitespace removal.

[pierre-rouleau]

I also noticed that a couple of forks have made modifications and additions. Mainly the ones from:

• felipeagc
• nverno
• ymarkovitch The last one added customization and auto-loaded them - I didn't do that because I use a mechanism to access customization group and prompt for loading their library if its not already done. It might be a good idea to inspect all forks and merge what works best. For the moment I don't really have time for that though.

[Wilfred]

Looks great, thank you!