stdenv: Store one package per line in nix-support/propagated-*

This makes those files a bit easier to read. Also, for what it's worth,
it brings us one baby step closer to handling spaces in store paths.

Also, I optimized handling of many transitive deps with read. Probably,
not very beneficial, but nice to enforce the pkg-per-line structure.
Doing so let me find much dubious code and fix it.

Two misc notes:

 - `propagated-user-env-packages` also needed to be adjusted as
   sometimes it is copied to/from the propagated input files.

 - `local fd` should ensure that file descriptors aren't clobbered
   during recursion.

pkgs/stdenv/generic/builder.sh

@@ -1,4 +1,3 @@
-export PATH=
 for i in $initialPath; do
     if [ "$i" = / ]; then i=; fi
     PATH=$PATH${PATH:+:}$i/bin
@@ -15,4 +14,6 @@ cat "$setup" >> $out/setup
 # Allow the user to install stdenv using nix-env and get the packages
 # in stdenv.
 mkdir $out/nix-support
-echo $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages
+if [ "$propagatedUserEnvPkgs" ]; then
+    printf '%s\n' $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages
+fi