maint: Use "m_" prefix for class member variables in load-path.[h|cc].

Follow Octave C++ coding guidelines and use "m_" prefix for member
variables even for private classes defined within another class.

* load-path.h (class dir_info, class file_info, class package_info):
Use 'm_' prefix for member variables.

* load-path.cc: Rename all necessary instances of variable names
modified in load-path.h.

Author: Rik