path.c

extensive change then what was suggested. I found the file path.c that
contained a lot of "Unix/Windows" agnostic functions so I added a function
there instead and removed the PATHSEP declaration in exec.c altogether. All
to keep things from scattering all over the code.

I also took the liberty of changing the name of the functions
"first_path_sep" and "last_path_sep". Where I come from (and I'm apparently
not alone given the former macro name PATHSEP), they should be called
"first_dir_sep" and "last_dir_sep". The new function I introduced, that
actually finds path separators, is now the "first_path_sep". The patch
contains changes on all affected places of course.

I also changed the documentation on dynamic_library_path to reflect the
chagnes.

Thomas Hallgren