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/setup.sh

@@ -215,6 +215,11 @@ isScript() {
     if [[ "$magic" =~ \#! ]]; then return 0; else return 1; fi
 }
 
+# printf unfortunately will print a trailing newline regardless
+printLines() {
+    [[ $# -gt 0 ]] || return 0
+    printf '%s\n' "$@"
+}
 
 ######################################################################
 # Initialisation.
@@ -300,9 +305,12 @@ findInputs() {
     fi
 
     if [ -f "$pkg/nix-support/$propagatedBuildInputsFile" ]; then
-        for i in $(cat "$pkg/nix-support/$propagatedBuildInputsFile"); do
-            findInputs "$i" $var $propagatedBuildInputsFile
+        local fd pkgNext
+        exec {fd}<"$pkg/nix-support/$propagatedBuildInputsFile"
+        while IFS= read -r -u $fd pkgNext; do
+            findInputs "$pkgNext" $var $propagatedBuildInputsFile
         done
+        exec {fd}<&-
     fi
 }
 
@@ -794,17 +802,17 @@ fixupPhase() {
         fi
         if [ -n "$propagated" ]; then
             mkdir -p "${!outputDev}/nix-support"
-            echo "$propagated" > "${!outputDev}/nix-support/propagated-native-build-inputs"
+            printLines $propagated > "${!outputDev}/nix-support/propagated-native-build-inputs"
         fi
     else
         if [ -n "$propagatedBuildInputs" ]; then
             mkdir -p "${!outputDev}/nix-support"
-            echo "$propagatedBuildInputs" > "${!outputDev}/nix-support/propagated-build-inputs"
+            printLines $propagatedBuildInputs > "${!outputDev}/nix-support/propagated-build-inputs"
         fi
 
         if [ -n "$propagatedNativeBuildInputs" ]; then
             mkdir -p "${!outputDev}/nix-support"
-            echo "$propagatedNativeBuildInputs" > "${!outputDev}/nix-support/propagated-native-build-inputs"
+            printLines $propagatedNativeBuildInputs > "${!outputDev}/nix-support/propagated-native-build-inputs"
         fi
     fi
 
@@ -817,7 +825,7 @@ fixupPhase() {
 
     if [ -n "$propagatedUserEnvPkgs" ]; then
         mkdir -p "${!outputBin}/nix-support"
-        echo "$propagatedUserEnvPkgs" > "${!outputBin}/nix-support/propagated-user-env-packages"
+        printLines $propagatedUserEnvPkgs > "${!outputBin}/nix-support/propagated-user-env-packages"
     fi
 
     runHook postFixup