Don't include sys/file.h for winnt; use process.h instead.

(concat): Now just concats two things; all callers changed.
(concat[346]): New functions.
(dir_separator_str): New variable.
(exec_prefixes, startfile_prefixes, include_prefixes): Renamed from same names
without plural; all users changed.
(find_a_file, process_command, do_spec_1): Use DIR_SEPARATOR.
(find_file, is_directory, main): Likewise.
(lookup_compiler): If OS/2, do case-independent search.
(fix_argv): New function on winnt.
(FIX_ARGV): New macro for some systems; use instead of argv.

From-SVN: r8184