Closes GH-5758

Even if that header file is available, we better consider it private,
and don't include it.  The information about whether SSL support is
enabled is now missing (`USE_(OPEN)SSL`), and it seems there is no
alternative way to get it (`PQinitSSL()` is always defined), so we
remove it from the PHP info.  Furthermore, the `PG_VERSION` and
`PG_VERSION_STR` macros are no longer available, but as of libpq 9.1
there is `PQlibVersion()` which allows us to construct `PG_VERSION` in
a most likely backwards compatible manner.  The additional information
available through `PG_VERSION_STR` is lost, though, so we define
`PGSQL_LIBPQ_VERSION_STR` basically as alias of `PGSQL_LIBPQ_VERSION`,
and deprecate it right away.

Since we are now requiring at least libpq 9.1, we can remove some
further compatibility code and additional checks.

Regarding the raised requirements: official support for PostGreSQL 9.0
ended on 2015-10-08, and even CentOS 7 already has PostGreSQL 9.2, so
this is not supposed to be too much of an issue.

* PHP-7.4:
  Fix #79532: sizeof off_t can be wrong

We have to actually determine the proper `SIZEOF_OFF_T`.
Interestingly, it is `4` on Windows x64.

We also have to prevent the redefinition in pg_config.h.  The clean
solution would likely be to not include pg_config.h at all, but that's
out of scope for BC reasons for now.

Closes GH-5066 As a first step, let's capitalize their initial letter when it is applicable.

Except for some bigger ones: reflection, sodium, spl