Make sure /dev/pts/ptmx is world-writable

While running Python 3’s test suite, we noticed that on some systems
/dev/pts/ptmx is created with permissions 0 (that’s the case with my
Nixpkgs-originating 3.0.43 kernel, but someone with a Debian-originating
3.10-3 reported not having this problem.)

There’s still the problem that people without
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y are screwed (as noted in build.cc),
but I don’t see how we could work around it.

src/libstore/build.cc

@@ -2086,6 +2086,10 @@ void DerivationGoal::initChild()
                 if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
                     throw SysError("mounting /dev/pts");
                 createSymlink("/dev/pts/ptmx", chrootRootDir + "/dev/ptmx");
+
+		/* Make sure /dev/pts/ptmx is world-writable.  With some
+		   Linux versions, it is created with permissions 0.  */
+		chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
             }
 
             /* Do the chroot().  Below we do a chdir() to the