smtp: handle DATA mode in middle of input parsing

smtp: handle DATA mode in middle of input parsing

Before:
If the input was such that we'd enter DATA mode in the middle, the
entire data would be passed through SMTPGetLine fn and be processed with
line limits etc in place.

After:
Since we don't want any limits to be enforced on DATA, we pass it to
SMTPPreProcessCommands fn to take care of it differently from the
commands.

Bug 5981