Revise representation of header-field objects, so that RFC-822 quoting

Revise representation of header-field objects, so that RFC-822 quoting
mechanism isn't part of the representation.  Value string of a header
is now arbitrary text, and there is an explicit mechanism for
converting the header value to RFC-822 format, by prepending tabs onto
continuation lines.

Additionally, there should no longer be any possibility of a low-level
error being generating when parsing header fields of incoming messages.