Attempt to move the POSIX iconv* symbols out of runtime linker space.
FreeBSD systems usually implemented this as a third party module and
our implementation hasn't played as nicely with the old way as it could
have.

To that end:
* Rename the iconv* symbols in libc.so.7 to have a __bsd_ prefix.
* Provide .symver compatability with existing 10.x+ binaries that
  referenced the iconv symbols. All existing binaries should work.
* Like on Linux/glibc systems, add a libc_nonshared.a to the ldscript
  at /usr/lib/libc.so.
* Move the "iconv*" wrapper symbols to libc_nonshared.a

This should solve the runtime ambiguity about which symbols resolve
to where.  If you compile against the iconv in libc, your runtime
dependencies will be unambiguous.

Old 9.x libraries and binaries will always resolve against their
libiconv.so.3 like they did on 9.x.  They won't resolve against libc.

Old 10.x binaries will be satisified by the .symver helpers.

This should allow ports to selectively compile against the libiconv
port if needed and it should behave without ambiguity now.

Discussed with:	 kib
Bump __FreeBSD_version for iconv changes
Fix the build of some ATF tests.

When building various programs from a single Makefile, program-specific
variables are of the form <VAR>.<PROG>, not <VAR>_<PROG>.  Fix this
obvious typo to fix the build when WITH_TESTS=yes.

I am not sure how this ever worked before given that manual inspection
of bsd.progs.mk clearly shows that the expected character between the
two components is a dot and not an underscore... but I suspect the
changes in r258095 exposed this oddity.

Approved by:	rpaulo (mentor)
Update notes for imports of atf.

This is because the atf vendor branch now includes a verbatim copy of
the distfile sources.  As a result, the list of files to-be-removed from
the contrib/ directory is now more aggressive (and different) and the
upgrade notes now only describe stuff that is specific to the atf import
and is not documented in the Subversion Primer.

Approved by:	rpaulo (mentor)