[9fans] Unescaped "From" in mail items

[quanstro@coraid.com (erik quanstrom)]

> This is acme mail's fault.  The easiest fix is to change
> /acme/mail/src/mesg.c to do two writes: first use write
> to write the header, and then use write2 to write the body
> (write2 is in reply.c and knows how to escape From).
> You'll probably also want to make write2 return whether
> or not it succeeded.

it's a bit odd that the mail readers (Mail, ned, imap4d)
all use upas/fs to take care of reading mailboxes, but
when appending a message to a mailbox, all three
assume they know how mailboxes are structured.

- erik