tests: Minor refactoring in tests/check_client.c

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
client: Implement sending messages over UDP too

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Support sending over UDP too

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>