Fix Libprocess benchmark OSX

pipe2 does not work on osx.
Replace pipe2 with a helper function that sets up the pipes using pipe()
and then stout/os::cloexec().

Review: https://reviews.apache.org/r/27116
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.
Discard label/tag when parsing Version string.

Review: https://reviews.apache.org/r/27115