doc: Improve suggested Emacs settings for our coding style.

Switch from changing global values to defining a separate C style
and add appropriate settings for indenting assignments that span
more than one line.

doc/developer.texi

@@ -226,10 +226,16 @@ autocmd InsertEnter * match ForbiddenWhitespace /\t\|\s\+\%#\@@<!$/
 
 For Emacs, add these roughly equivalent lines to your @file{.emacs.d/init.el}:
 @example
-(setq c-default-style "k&r")
+(c-add-style "libav"
-(setq-default c-basic-offset 4)
+             '("k&r"
-(setq-default indent-tabs-mode nil)
+               (c-basic-offset . 4)
-(setq-default show-trailing-whitespace t)
+               (indent-tabs-mode nil)
+               (show-trailing-whitespace t)
+               (c-offsets-alist
+                (statement-cont . (c-lineup-assignments +)))
+               )
+             )
+(setq c-default-style "libav")
 @end example
 
 @section Development Policy