openjdk: Introduce JAVAX_NET_SSL_TRUSTSTORE env

This small patch makes it possible to control java's truststore path through
the environment. This lets you add (system- or session-wide) CAs that should
be allowed by Java. Java users can still use -Djavax.net.ssl.truststore to
override the truststore set by JAVAX_NET_SSL_TRUSTSTORE.

Something like this can be used to build the truststore (in this example just
using the standard pkgs.cacert CA-bundle):

{
environment.variables.JAVAX_NET_SSL_TRUSTSTORE = "${
pkgs.runCommand "cacerts" {} ''
${pkgs.perl}/bin/perl \
${pkgs.path}/pkgs/development/compilers/openjdk/generate-cacerts.pl \
${pkgs.jre}/bin/keytool \
${pkgs.cacert}/etc/ca-bundle.crt
mv cacerts $out
''
}";
}

Ideally, the dependency on pkgs.cacert should also be removed from pkgs.openjdk
to avoid rebuilding java each time the standard CA-bundle changes. Something
along the example above must then be added to NixOS (however, it would be
nice to not depend on ${pkgs.jre}/bin/keytool to generate that environment
variable).

pkgs/development/compilers/openjdk/default.nix

···
     makeFlagsArray+=(CUPS_HEADERS_PATH=$cupsDir)
   '';
 
-  patches = [ ./cppflags-include-fix.patch ./fix-java-home.patch ./paxctl.patch ];
+  patches = [
+    ./cppflags-include-fix.patch
+    ./fix-java-home.patch
+    ./paxctl.patch
+    ./read-truststore-from-env.patch
+  ];
 
   NIX_NO_SELF_RPATH = true;
 

pkgs/development/compilers/openjdk/read-truststore-from-env.patch

···
+diff -ur openjdk-7u65-b32/jdk/src/share/classes/sun/security/ssl/TrustManagerFactoryImpl.java openjdk-7u65-b32.new/jdk/src/share/classes/sun/security/ssl/TrustManagerFactoryImpl.java
+--- openjdk-7u65-b32/jdk/src/share/classes/sun/security/ssl/TrustManagerFactoryImpl.java	2014-07-17 12:12:14.000000000 +0200
++++ openjdk-7u65-b32.new/jdk/src/share/classes/sun/security/ssl/TrustManagerFactoryImpl.java	2014-12-09 13:31:27.821960372 +0100
+@@ -158,6 +158,7 @@
+         /*
+          * Try:
+          *      javax.net.ssl.trustStore  (if this variable exists, stop)
++         *      system environment variable JAVAX_NET_SSL_TRUSTSTORE
+          *      jssecacerts
+          *      cacerts
+          *
+@@ -165,6 +166,9 @@
+          */
+ 
+         storeFileName = props.get("trustStore");
++        if (storeFileName == null) {
++            storeFileName = System.getenv("JAVAX_NET_SSL_TRUSTSTORE");
++        }
+         if (!"NONE".equals(storeFileName)) {
+             if (storeFileName != null) {
+                 storeFile = new File(storeFileName);