stdenv.isBSD: reinit

This was removed in e29b0da9c7, because
it was felt it was ambiguous whether isBSD should remove Darwin.

I think it should be reintroduced.  Packages sometimes have their own
concepts of "is BSD" e.g. Lua, and these almost never include Darwin,
so let's keep Darwin excluded.

Without a way to say "is this BSD", one has to list all flavours of
BSD seperately, even though fundamentally they're still extremely
similar.  I don't want to have to write the following!

    stdenv.isFreeBSD || stdenv.isNetBSD || stdenv.isOpenBSD || stdenv.isDragonFlyBSD

Additionally, we've had stdenv.hostPlatform.isBSD this whole time, and
it hasn't hurt anything.

pkgs/stdenv/generic/default.nix

@@ -137,7 +137,7 @@ let
 
       # Utility flags to test the type of platform.
       inherit (hostPlatform)
-        isDarwin isLinux isSunOS isCygwin isFreeBSD isOpenBSD
+        isDarwin isLinux isSunOS isCygwin isBSD isFreeBSD isOpenBSD
         isi686 isx86_32 isx86_64
         is32bit is64bit
         isAarch32 isAarch64 isMips isBigEndian;

pkgs/tools/misc/ffsend/default.nix

@@ -7,7 +7,7 @@
 }:
 
 let
-  usesX11 = stdenv.isLinux || stdenv.hostPlatform.isBSD;
+  usesX11 = stdenv.isLinux || stdenv.isBSD;
 in
 
 assert (x11Support && usesX11) -> xclip != null || xsel != null;