[pdal] Entwine directory permissions on Linux

[Jim Klassen]

Is there a reason Entwine (actually arbiter) creates directories with mask 744 on Linux instead of 755 or just using 777 and letting the user control the actual permissions with their umask?  744 seems particularly strange (vs 700 or 755) as read w/o execute on a directory doesn't get you much.

The line in question is arbiter.cpp:1094 in mkdirp(std::string raw):

 #ifndef ARBITER_WINDOWS
        const bool err(::mkdir(cur.c_str(), S_IRWXU | S_IRGRP | S_IROTH));
         if (err && errno != EEXIST) return false;
 #else