[Exim] Second stab at supporting Cyrus SASL's saslauthd

Hello.

This patch against 4.20 adds the saslauthd condition, which is like pwcheck,
but not entirely.

Syntax is something like

${if saslauthd{{username}{password}{service}{realm}}{yea}{nay}}

service and realm parameters are optional.
I suspect the most used form will be something like:

server_condition = ${if saslauthd{{$2}{$3}}{1}{0}}

The condition is configure in Local/Makefile just like pwcheck -
with

CYRUS_SASLAUTHD_SOCKET=/var/blah/foo

We haven't yet deployed Cyrus IMAP, so the testing was at most cursory.

With sub-curly-braces I got rid of possibility of encountering ':' in password
or elsewhere (by which i was once bitten), but now none of the fields can
contain '{' or '}'. Not that curly braces are that common in passwords, but
nevertheless is there any way to somehow escape curly braces?

--

./lxnt