Issue 3559: Use From or ReplyTo hostname in Message-ID if available.

I wrote this fix to avoid obviously specifying that I am using a mobile device
to reply to an email.

Others want this for ease of filtering messages from their host by Message-ID.
Issue 4690: Add privacy pref to omit K-9 User-Agent header.
Issue 6372: Add preference for setting timezone to UTC.

This is a privacy preference to avoid leaking your current location while
replying to email.
Fixes for cketti's code review on pull req #472

Leave the hostname == null checks so we can fall back if a hostname is not
found. Also convert message-id to upper case to match Apple Mail (for
privacy).
Merge pull request #472 from mikeperry-tor/improve-header-privacy

Improve header privacy (Issues 6372, 3559, and 4690)