Revert "mime_alter: Don't try to force insert pre html"

This reverts commit ba5b83209fac154493e1dfb63048a3610025f448.

Client wants to force insert pre-html. Lets go with this simple solution
of if the first line in the mime part isnt <body> it will force insert
the pre-html.

Client says that most of the HTML email they need to process doesn't
have html or body taga, so this should generally be OK.
mime_alter: Prevent duplicate lines when force inserting pre-html

After commit 15b3a1c ("mime_alter: Fix HTML insertion") we could end up
with a repeated line in the output. e.g

    <br>
    <b>Hello World</b>
    <br>

    <pre>
    <pre>	<- duplicated
    Some html...
    </pre>

    <br>
    <b>Goodbye</b>
    <br>

This was due to adding in the last line read after force inserting the
pre-html.

This line was already going to be added by the final fprintf() which was
there to ensure that the mime part boundary got written.

So remove the fprintf() from the force insertion bit.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>
ffget: Fix the name of FFGET_seek in ffget.h

It was down as FFGET_fseek.

We will want to use this function in a subsequent commit.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>
Introduce --{pretext,disclaimer}-html-b64= options

The purpose of these is to allow you to insert BASE64 encoded HTML into
BASE64 encoded text/html MIME parts.

In trying to not introduce too much code duplication while also trying
not to break what was already working. I made a new function
AM_insert_HTML_disclaimer_into_segment64() based on the HTML handling
part of AM_insert_disclaimer_into_segment64(). The main difference here
is that this uses AM_add_disclaimer_insert_html() rather than
AM_insert_HTML_disclaimer_into_buffer().

I wanted to make use of the changes already made to
AM_add_disclaimer_insert_html for the forcing of pre and post html and
this function only required minor modifications to make it suitable for
re-use.

So now you can use --pretext-html-b64= & --disclaimer-html-b64=
or just go for the full thing with something like

    --pretext= --pretext-html= --pretext-b64= --pretext-html-b64=
    --disclaimer= --disclaimer-html= --disclaimer-b64=
    --disclaimer-html-b64= --force-for-bad-html --input=

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>
qpe: Add a missing \n to a debug statement

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>