Deprecating non-functional SHAREDSTRINGS build option and xstr(1)

I'm going to deprecate the build option SHAREDSTRINGS in bsd.prog.mk
and any other associated build machinery, and probably remove
xstr(1) too.

Unless, of course, there's an exceptionally good reason to keep this
non functional feature.


Rationale:

1. The build option SHAREDSTRINGS hasn't worked for at least 16 years,
and possibly even longer, maybe as far back as 2003-07-22 when
I added .c.o rules to bsd.sys.mk (so the SHAREDSTRINGS .c.o rules
in bsd.prog.mk aren't actually being used).

See PR 35964 at
	https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=35964

Even though gnu/dist/bc was replaced with usr.bin/dc, building -current
still fails (at least in tests/bin/df and other tests/ subdirs that
build PROG), and the workaround looks ugly.


2. xstr is not a host tool, probably because it doesn't work as a native
build option.


3. The problem that xstr(1) solves is better solved by modern compilers and
linkers, which have evolved in the 30+ years that xstr has existed.


Thoughts?

Luke.