fish: add module to support it as default shell

* Patched fish to load /etc/fish/config.fish if it exists (by default,
  it only loads config relative to itself)
* Added fish-foreign-env package to parse the system environment

closes #5331

pkgs/shells/fish/default.nix

@@ -1,4 +1,5 @@
-{ stdenv, fetchurl, ncurses, nettools, python, which, groff, gettext, man_db, bc, libiconv, coreutils }:
+{ stdenv, fetchurl, ncurses, nettools, python, which, groff, gettext, man_db,
+  bc, libiconv, coreutils, gnused, kbd }:
 
 stdenv.mkDerivation rec {
   name = "fish-${version}";
@@ -9,6 +10,9 @@ stdenv.mkDerivation rec {
     sha256 = "0ympqz7llmf0hafxwglykplw6j5cz82yhlrw50lw4bnf2kykjqx7";
   };
 
+  # builtin_status has been upstreamed https://github.com/fish-shell/fish-shell/pull/2636
+  patches = [ ./etc_config.patch ./builtin_status.patch ];
+
   buildInputs = [ ncurses libiconv ];
 
   # Required binaries during execution
@@ -18,15 +22,26 @@ stdenv.mkDerivation rec {
                           ++ [ bc coreutils ];
 
   postInstall = ''
+    sed -e "s|expr|${coreutils}/bin/expr|" \
+        -e "s|if which unicode_start|if true|" \
+        -e "s|unicode_start|${kbd}/bin/unicode_start|" \
+        -i "$out/etc/fish/config.fish"
     sed -e "s|bc|${bc}/bin/bc|" \
         -e "s|/usr/bin/seq|${coreutils}/bin/seq|" \
         -i "$out/share/fish/functions/seq.fish" \
            "$out/share/fish/functions/math.fish"
     sed -i "s|which |${which}/bin/which |" "$out/share/fish/functions/type.fish"
+    sed -e "s|\|cut|\|${coreutils}/bin/cut|" -i "$out/share/fish/functions/fish_prompt.fish"
     sed -i "s|nroff |${groff}/bin/nroff |" "$out/share/fish/functions/__fish_print_help.fish"
     sed -e "s|gettext |${gettext}/bin/gettext |" \
         -e "s|which |${which}/bin/which |" \
         -i "$out/share/fish/functions/_.fish"
+    sed -e "s|uname|${coreutils}/bin/uname|" \
+        -i "$out/share/fish/functions/__fish_pwd.fish" \
+           "$out/share/fish/functions/prompt_pwd.fish"
+    sed -e "s|sed |${gnused}/bin/sed |" \
+        -i "$out/share/fish/functions/alias.fish" \
+           "$out/share/fish/functions/prompt_pwd.fish"
     substituteInPlace "$out/share/fish/functions/fish_default_key_bindings.fish" \
       --replace "clear;" "${ncurses}/bin/clear;"
   '' + stdenv.lib.optionalString (!stdenv.isDarwin) ''