doc: Update coding style

Add a new section for troff style. Add a subsection for C++ extensions.
Update perl style to mention that the development checks need to pass,
that private modules can use POD but need to be marked as such, and
to update indentation and alignment to use soft tabs only.

doc/coding-style.txt

@@ -1,5 +1,21 @@
-Dpkg C coding style 2012-05-24
-===================
+Dpkg troff coding style 2016-01-29
+=======================
+
+General
+~~~~~~~
+
+Dashes that are relevant when copy & pasted need to be escaped (e.g. those
+present in program, file, argument and field names).
+
+New sentences inside a paragraph should start on a new line, so that we
+do not need to reflow the text when adding new content.
+
+Every new feature, option or behavior change needs to be documented with
+the version introducing the change.
+
+
+Dpkg C/C++ coding style 2016-01-29
+=======================
 
 C language extensions
 ~~~~~~~~~~~~~~~~~~~~~
@@ -17,6 +33,16 @@ The code base assumes C89 plus the following C99 extensions:
 Those are checked at build time, and it will abort in case a needed extension
 is not supported.
 
+C++ language extensions
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The code base assumes C++03 plus the following C++11 extension:
+
+ * Null pointer keyword (nullptr).
+
+This is checked at build time, and it will use compatibility code in case the
+needed extension is not supported.
+
 General
 ~~~~~~~
 
@@ -207,35 +233,37 @@ see what operation is being done:
 		foo();
 
 
-Dpkg Perl coding style 2015-05-20
+Dpkg Perl coding style 2016-01-29
 ======================
 
 General
 ~~~~~~~
 
 In general you should follow the conventions listed in perlstyle(1).
-All the code should run with the “use strict” and “use warnings” pragmas.
+All the code should run with the “use strict” and “use warnings” pragmas,
+and pass «DPKG_DEVEL_MODE=1 make check».
 
 Code documentation
 ~~~~~~~~~~~~~~~~~~
 
 Public modules should be documented with POD (see perlpod(1)). Private
 code doesn't have to use POD, simple comment lines (starting with "#") are
-enough. Public scripts are documented in their corresponding manual pages.
+enough, but if they use POD they need to note the fact that the module is
+private in the CHANGES section and specify a version «0.xx». Public scripts
+are documented in their corresponding manual pages.
 
 Indentation, alignment and spacing
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Lines should be 80 chars max. The indentation level is 4 characters, and
-indentation is done with hard tabs (which should be considered to take 8
-spaces width) and spaces.
+indentation is done with soft tabs (no hard tabs) and spaces.
 
 if ($foo) {
     if ($bar) {
-	print "Hello\n";
-	unless ($baz) {
-	    print "Who are you?\n";
-	}
+        print "Hello\n";
+        unless ($baz) {
+            print "Who are you?\n";
+        }
     }
 }