On the smtp client package, there is no limit to the length of the
server's replies, so an evil server could cause a memory exhaustion DoS
by issuing very long lines.

This patch fixes the bug by limiting the total size of received data.
Ideally this would be done per-line instead, but gets much more complex,
so this is a compromise.

The limit chosen is 2 MiB, which should be plenty for any the total size
of server-side replies, considering we only send a single message per
connection anyway.

This is similar to 06d808c8 (smtpsrv: Limit incoming line length), which
was found and reported by Max Mazurov (fox.cpp@disroot.org).