* dir.cc (mkdir): Check for case clash.

    * dir.cc (mkdir): Check for case clash.
        * environ.cc: Add extern declaration for `pcheck_case'.
        (check_case_init): New function.
        (struct parse_thing): Add "check_case" option.
        * errno.cc (_sys_nerrlist): Add text for ECASECLASH.
        (strerror): Add case branch for ECASECLASH.
        * fhandler.cc (fhandler_disk_file::open): Check for case clash.
        * path.cc: Add global variable `pcheck_case'.
        (struct symlink_info): Add member `case_clash' and method `case_check'.
        (path_prefix_p_): Call `pathnmatch' instead of `strncasematch'.
        (pathnmatch): New funtion.
        (pathmatch): Ditto.
        (path_conv::check): Add handling for case checking.
        (symlink): Check for case clash.
        (symlink_info::check): Add parameter for case checking.
        Handle case checking.
        (symlink_info::case_check): New method.
        (chdir): Don't use unconverted path if pcheck_case==PCHECK_STRICT.
        * path.h: Add extern declarations for `pathmatch' and
        `pathnmatch'.
        (enum case_checking): New enumeration type describing
        the case checking behaviour of path conversion routines.
        (class path_conv): Add member `case_clash'.
        * syscalls.cc (_link): Check for case clash.