inboxwritable: fix From_ line unescaping

inboxwritable: fix From_ line unescaping

We can't rely on Email::MIME noticing the change to our
scalar ref after calling `PublicInbox::MIME->new'.

This is because Email::MIME::body_set (unlike
Email::Simple::body_set) will copy the contents of the body into
`->{body_raw}' as a new scalar.

Furthermore, we need to escape multiple From lines in the body,
not just the first one, using the `g' modifier to `s//'.

Reported-by: Kyle Meyer <kyle@kyleam.com>