Better testing of float `to_string` lengths.

More portable ways of trying to hit the worst-case string length for
floating-point conversions to strings.  Thanks @GordonLElliott.

Fix link error. Backported from 7.0.

Fixes jtv#340.

A missing `PQXX_LIBEXPORT` could cause link errors for a missing
`extract_value` specialisation for `nullptr_t`, when building libpqxx as
a shared library.

Add a missing header to install.

Fixes jtv#320.

Cosmetic.

Add bug number to NEWS.

Fix more handling of bad float strings.

Fixes jtv#308.

Add `esc(std::string_view)`.

Fixes jtv#295.

I guess I meant to do this before, but forgot!  The overload was there
on `connection` but not on `transaction_base`.

Also add missing operators on row iterator.

Fix handling of eof() in streambuffer underflow.

@tomlankhorst has identified a serious bug which affected the
`ilostream` class.  If you opened a large object _as an ilostream,_
and it contained a byte with value `0xff` _at a buffer boundary,_
on a system where `char` is a signed type (such as x86-compatible
systems), then reading of the large object would **mistake the
`0xff` byte for end-of-file** and stop reading prematurely.

Update large objects for house style. Fix types.