Fix the communication in qPlatform_[mkdir,chmod]

With the previous implementation the protocol used by the client and the
server for the response was different and worked only by an accident.
With this change the communication is fixed and the return code from
mkdir and chmod correctly captured by lldb. The change also add
documentation for the qPlatform__[mkdir,chmod] packages.

Differential revision: http://reviews.llvm.org/D7786

llvm-svn: 230213