Add a -w warning flag to ln(1). When the -w option is enabled,

Add a -w warning flag to ln(1).  When the -w option is enabled,
ln(1) checks to see if the source of a symlink, i.e. the file it
should point to actually exists.  The default is the old ln
behavior, that does not check, to avoid surprising people who may
be using ln(1) in scripts or other non-interactive places.

PR:		bin/7265
Submitted by:	Joel Ray Holveck, detlev!joelh at mail.camalott.com
MFC after:	2 weeks