(cygheap_fdmanip::isopen): New method.
* syscalls.cc (_read): Avoid accessing closed fd.
* path.h (fe_types): New enum.
(path_conv::set_path): New method.
(find_exec): Change null_if_not_found argument to something more generic.
* spawn.cc (find_exec): Default to returning the POSIX path rather than the
windows path, unless instructed otherwise.
(spawn_guts): Force call to find_exec to use native paths.
* dlfcn.cc (check_path_access): Accommodate new find_exec arguments.
* environ.h (win_env::get_posix): New method.
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