client: Add a riemann_client_get_fd() function

This function can be used to get thet file descriptor behind a Riemann
client connection. The intended use is for setting various TCP or socket
optionss (such as TCP_CORK) on it from the application.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
docs: Add a short paragraph about riemann_client_get_fd()

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Move config.h to lib/riemann/platform.h

Use "riemann/platform.h" instead of config.h, to avoid potential
filename conflicts.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Add a --version command-line option

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Add --version to the subcommands too

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