LogProtoIndentedMultilineServer: introduce new indented-multiline protocol

This new protocol implementation allows the use of multiline messages,
by treating indented lines as continuations of the last line. This format
is being introduced in /dev/kmsg.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Makefile.am: compile libtest first so that tests under lib work

The tests for logproto moved to lib/logproto/tests, which require the
test support code, so compile that first.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>