From an engine perspective, named parameters mainly add three
concepts:

 * The SEND_* opcodes now accept a CONST op2, which is the
   argument name. For now, it is looked up by linear scan and
   runtime cached.
 * This may leave UNDEF arguments on the stack. To avoid having
   to deal with them in other places, a CHECK_UNDEF_ARGS opcode
   is used to either replace them with defaults, or error.
 * For variadic functions, EX(extra_named_params) are collected
   and need to be freed based on ZEND_CALL_HAS_EXTRA_NAMED_PARAMS.

RFC: https://wiki.php.net/rfc/named_params

Closes GH-5357.

And make sure too few/many arguments throws, to satisfy the
function signature.

These functions don't do anything special, just use normal zpp.

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.

We can safely assume that users have at the very least libpq 7.4, for
which official support ended on 2010-10-01; even CentOS 6 has 8.4 now.
It is also noteworthy that PDO_PGSQL already requires libpq 7.4 or
later.

From now on, float to string casting will always behave locale-independently.
RFC: https://wiki.php.net/rfc/locale_independent_float_to_string
Closes GH-5224

Co-authored-by: George Peter Banyard <girgias@php.net>