rfc822header: improvements to render attribute on headers.

rfc822header: improvements to render attribute on headers.

1/ the render:rfc822header-FOO attr is now render:rfc822header:FOO
   to differentiate from e.g. render:rfc822header-wrap which doesn't
   identify a header.
2/ The "FOO" in the above is now always lower-case.  This removes the
   need to differentiate between hdr and hdr_found
3/ The value of that attr is the type of header.  This will be used
    more later.
4/ change notmuch to make proper use of some of the above.

Signed-off-by: NeilBrown <neil@brown.name>