maint: Revamp DEBUG code for kpse subsystem.

* kpse.cc: Replace all different debug #defines with just one:
OCTAVE_KPSE_DEBUG.  Delete static variable kpse_debug previously
used to dynamically track debug state.  Replace C-style comments
(/* ... */) that were only one line with C++ style comments (//).
Wrap some long comment lines to 80 characters rather than 72.
Replace uses of "KPSE_DEBUG_P (KPSE_DEBUG_SEARCH)" with "OCTAVE_KPSE_DEBUG".

* pathsearch.cc (directory_path::init): Remove code that dynamically checked
environment variable KPATHSEA_DEBUG and set internal static variable
kpse_debug.

Author: Rik