bug#26923: 25.2; `setq-local': allow multiple assignments or add optional buffer arg

[Drew Adams]

1. I believe this has been discussed before, and presumably the result
   (decision or lack of one) has been not to make the change, but I'd
   still suggest that `setq-local', like `setq', should allow multiple
   assignment pairs.

2. If that's definitely rejected then please consider this other
   enhancement instead: Allow a buffer argument:

   (setq-local VAR VALUE &optional BUFFER)

   This would be essentially just a convenience.

   See this emacs.SE question:
   https://emacs.stackexchange.com/q/32787/105

In GNU Emacs 25.2.1 (x86_64-w64-mingw32)
 of 2017-04-24
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --without-compress-install 'CFLAGS=-O2
 -static -g3''