* newlib/libc/include/sys/unistd.h: Add prototypes for

    * newlib/libc/include/sys/unistd.h: Add prototypes for
        fchmod, fchown, lchown.
        * winsup/cygwin/syscalls.cc (chown_worker): Use previous
        uid/gid if new uid/gid is -1.
        New static function with chown functionality.
        (chown): Call chown_worker with SYMLINK_FOLLOW.
        (fchown): New function. Call chown_worker with SYMLINK_FOLLOW.
        (lchown): New function. Call chown_worker with SYMLINK_IGNORE.
        * cygwin.din: Add symbols for fchown, lchown.
        * path.cc (symlink): Call `set_file_attribute()' and
        `SetFileAttributeA()' instead of `chmod()' to set
        uid/gid correct.