The XCLIENT command uses xtext encoding for attribute values,
as specified in https://www.postfix.org/XCLIENT_README.html.

Reported by Igor Morgenstern of Aisle Research.

There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>

Previously, the default pool alignment used sizeof(unsigned long), with
the expectation that this would match to a platform word size.  Certain
64-bit platforms prove this assumption wrong by keeping the 32-bit long
type, which is fully compliant with the C standard.

This introduces a possibility of suboptimal misaligned access to the
data allocated with ngx_palloc() on the affected platforms, which is
addressed here by changing the default NGX_ALIGNMENT to a pointer size.

As we override the detection in auto/os/conf for all the machine types
except x86, and Unix-like 64-bit systems prefer the 64-bit long, the
impact of the change should be limited to Win64 x64.

When client address is received, IPv6 address could be specified without
square brackets and without port, as well as both with the brackets and
port.  The change allows IPv6 in square brackets and no port, which was
previously considered an error.  This format conforms to RFC 3986.

The change also affects proxy_bind and friends.