client: Implement network connections

This adds the ability to connect to Riemann. No sending or receiving
yet.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
client: Implement the TCP half of riemann_client_send_message()

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Implement network support and --help

To make the example tool easier to use, implement a help, and add
network support too, now that we have message sending over TCP working.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
configure: Add a few more checks

Based on the output of autoscan, added a few more checks.

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