man-db: make it the default man provider

For now, leave the old implementation under `man-old` attribute.

Small warning: I had a leftover ~/.nix-profile/man from an old package,
which caused man-db's man prefer it and ignore ~/.nix-profile/share/man.
The PATH->MANPATH code just selects the first match for each PATH item.

nixos/modules/programs/man.nix

···
 
   config = mkIf config.programs.man.enable {
 
-    environment.systemPackages = [ pkgs.man ];
+    environment.systemPackages = [ pkgs.man-db ];
 
     environment.pathsToLink = [ "/share/man" ];
 

pkgs/top-level/aliases.nix

···
   lttngTools = lttng-tools;  # added 2014-07-31
   lttngUst = lttng-ust;  # added 2014-07-31
   manpages = man-pages; # added 2015-12-06
+  man_db = man-db; # added 2016-05
+  man = man-db; # added 2016-05
   midoriWrapper = midori; # added 2015-01
   mlt-qt5 = qt5.mlt;  # added 2015-12-19
   module_init_tools = kmod; # added 2016-04-22

pkgs/top-level/all-packages.nix

···
 
   makemkv = callPackage ../applications/video/makemkv { };
 
-  man = callPackage ../tools/misc/man { };
+  man-old = callPackage ../tools/misc/man { };
 
-  man_db = callPackage ../tools/misc/man-db { };
+  man-db = callPackage ../tools/misc/man-db { };
 
   mawk = callPackage ../tools/text/mawk { };