ln: Allow a trailing slash when creating a link to a directory.

In the 'ln source... directory' synopsis, the basename of each source
determines the name of the created link. Determine this using basename(3)
instead of strrchr(..., '/') which is incorrect if the pathname ends in a
slash.

The patch is somewhat changed to allow for basename(3) implementations that
change the passed pathname, and to fix the -w option's checking also.
The code to compare directory entries only applies to hard links, which
cannot be created to directories using ln.

Example:

ln -s /etc/defaults/ /tmp

This should create a symlink named defaults.

PR: 121568
Submitted by: Ighighi
MFC after: 1 week