quote: fuse into quotearg

quote: fuse into quotearg

This patch is made for the benefit of Bison.
quote does not leave the choice of the quoting style to the user.
quoting_style provides poor customizability, yet quoting_options,
which is very rich, is hidden inside quotearg.c.  So in order to
allow quote customization, move its implementation to quotearg.c.
* lib/quote.c: Remove.
* modules/quote: Adjust.
* lib/quotearg.c (quoting_options_from_style): Fix a compiler
warning: provide all the members of literal structs.
(quote_quoting_options): New.
(quote, quote_n): Import implementation from quote.c.
* lib/quote.h: Import the comments from quote.c.
(quote_quoting_options): New.