drrun overwrites LD_PRELOAD when using -late

Created by: mattgreen

Describe the bug When using drrun's -late mode of injection, LD_PRELOAD is employed to load the DR support libraries. In the process of doing so, DR overwrites the existing value of LD_PRELOAD, rather than appending to the existing value.

To Reproduce

$ cd <DR_ROOT>/bin64
$ LD_PRELOAD=libtest.so ./drrun -late -c ../samples/bin64/libbbcount.so -- env

Note that the output does not include libtest.so.

Expected behavior LD_PRELOAD's value should be preserved through invocations of drrun.

Versions Occurs on 7.1 and 8.0 releases on Linux on both 32/64 bit builds.

Additional context PR coming shortly. Issue migrated from Google Groups discussion.