Version 3.33
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1. The test for an unset system-specific ERRNO_QUOTA was happening _before_ the
inclusion of config.h, where the system-specific setting happens. (I think it's
only SCO, which has no quotas, that actually sets this.) This caused a warning
about redefinition of the macro.

2. Change 3.32/7 broke IPv6 on Linux, which handles wildcard listening with a
single IPv6 socket, and _forbids_ a second IPv4 socket. (But the USAGI IPv6
stack may be different.) The change also caused failures on systems that have
IPv6 libraries, but no IPv6 support in the kernel. The IPv6 code has been
reworked yet again, such that it should work on all the different variations,
and just revert to IPv4 when there is no IPv6 support in the kernel.

3. Aliasing a local part to /dev/null without setting file_transport caused
Exim to crash. Now it gives the same error as any other /file alias.

4. As the IETF looks to be about to demote A6 DNS records to "experimental"
status, I have cut out their support, using a compile-time macro. This, of
course, applies only when Exim is built with IPv6 support.

5. Expanded error message for unknown rewrite flag item to suggest it might be
caused by missing quotes (because this is turning into a FAQ).

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