Revert "MESOS-1857 Fixed path::join() on older libstdc++ which lack back()."

This reverts commit ca2e8efcad7d312c6e93acd3546b6811a9f0df19.
Revert "Switched path::join() to be variadic".

This reverts commit b08fccf8f5ea325b8c38055b5f2c03509744dd9b.