Fix some POSIX-compliance bugs in link, rename, mkdir.

Fix some POSIX-compliance bugs in link, rename, mkdir.

* syscalls.cc (link): Delete obsolete comment.  Reject directories
and missing source up front.
(rename): Use correct errno for trailing '.'.  Detect empty
strings.  Allow trailing slash to newpath iff oldpath is
directory.
* dir.cc (mkdir): Reject dangling symlink with trailing slash.
* fhandler_disk_file.cc (fhandler_disk_file::link): Reject
trailing slash.
* fhandler.cc (fhandler_base::link): Match Linux errno.