complete 6855 input support

still needs to downgrade on output
Add support for RFC6531

This is dead code so far, since there's other code that doesn't
support 6531.
advertise EAI support for incoming mail
add UString::section()
make Address::localpart() and domain() return UString

update callers. This is certainly broken, since it depends on many
database columns to be of type citext, and it's a big enough change
that I expect it's broken for other reasons as well.
do not send autoreplies to *noreply* or mailer-daemon
send fewer autoreplies by default

before, two messages inbound with different Subject would both receive an
autoreply, since the default handle depended on the subject. now, the
default handle is empty, so all messages from the same sender have the
same handle, and the number of messages to that recipient will be limited
more strictly by default.
reenable THREAD=*, since the bug that broke them if fixed
Change to better/more modern cipher suite preferences.

The new list eliminates the medium-security cipher suites, since there's
wide support for high-security ciphers today.

The new list also prefers cipher suites with perfect forward security (ie.
ones that cannot be snooped) over other high-security suites. IMO Edward
Snowden has revealed that anything without PFS isn't really high security.
speling
handle addresses ins " æ <example@example.org>

those aren't terribly common, except in unit tests. eh.
valid() is invariant in this loop and need not be called
fix address deduplication

this was broken:
  To: A <a@example.com>, a@example.com
testing against a->base is an off-by-one (sometimes)
EAI support now compiles
make MimeField::rfc822 be an override again.

when HeaderField::rfc822 change signature, I forgot to handle the
mimefield reimplementation.
Use the schema with unicode-capable addresses
avoid extra forking of serverProcesses is one

this helps debugging, important if your name happens to be arnt
Treat localparts case-insensitively on injection

Now that we support unicode localparts, we have to do this. No longer can
we distinguish between Alex.Beta@example.com and alex.beta@example.com.

This change also removes an unnecessary call to lower().
Treat addresses properly case-insensitively when inserting many