logproto: Handle cases where writev() returns EAGAIN.

During log_prot_file_writer_flush, if writev() returns -1 and sets
errno to EAGAIN, do not die of an assert, but bubble an LPS_ERROR up
to the caller.

In log_proto_file_writer_post(), check the return value of
log_proto_file_writer_flush(), and handle the error case appropriately
(ie, undoing the consuming of the message).

Many thanks to Viktor Tusa for his initial debugging.

Signed-off-by: Viktor Tusa <tusa@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>