If we do multiple writev() calls and the last one fails, we shouldn't treat all of it as a failure because some data was already sent. Also added some more asserts.
If we do multiple writev() calls and the last one fails, we shouldn't treat
all of it as a failure because some data was already sent. Also added some
more asserts.

--HG--
branch : HEAD

dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot