Fix fork/exec stuff so it works again. We have to mangle LD_LIBRARY_PATH

Fix fork/exec stuff so it works again.  We have to mangle LD_LIBRARY_PATH
as well as LD_PRELOAD, so as to make our libpthread.so go out of scope
when a child which we don't want to trace, is exec'd.  Otherwise the
child can wind up being connected to our libpthread.so but not to
valgrind.so, which is an unworkable combination; you have to be connected
to both or neither.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@291