On ELF-based platforms (and maybe others?) it's possible for a shared
library, when dynamically loaded into the backend, to call the backend
versions of src/port and src/common functions rather than the frontend
versions that are actually linked into the shlib.  This is the cause
of bug #15367 from Jeremy Evans, and is likely to lead to more problems
in future; it's accidental that we've failed to notice any bad effects
up to now.

The recommended way to fix this on ELF-based platforms is to use a
linker "version script" that makes the shlib's versions of the functions
local.  (Apparently, -Bsymbolic would fix it as well, but with other
side effects that we don't want.)  Doing so has the additional benefit
that we can make sure the shlib only exposes the symbols that are meant
to be part of its API, and not ones that are just for cross-file
references within the shlib.  So we'd already been using a version
script for libpq on popular platforms, but it's now apparent that it's
necessary for correctness on every ELF-based platform.

Hence, add appropriate logic to the openbsd, freebsd, and netbsd stanzas
of Makefile.shlib; this is just a copy-and-paste from the linux stanza.
There may be additional work to do if commit ed0cdf0e reveals that the
problem exists elsewhere, but this is all that is known to be needed
right now.

Back-patch to v10 where SCRAM support came in.  The problem is ancient,
but analysis suggests that there were no really severe consequences
in older branches.  Hence, I won't take the risk of such a large change
in the build process for older branches.

In passing, remove a rather opaque comment about -Bsymbolic; I don't
think it's very on-point about why we don't use that, if indeed that's
what it's talking about at all.

Patch by me; thanks to Andrew Gierth for helping to diagnose the problem,
and for additional testing.

Discussion: https://postgr.es/m/153626613985.23143.4743626885618266803@wrigleys.postgresql.org