BUG/MINOR: checks: tcp-check actions are enums, not flags

In recent commit 5ecb77f (MEDIUM: checks: add send/expect tcp based check),
bitfields were mistakenly used at some places for the actions. Fortunately,
the only two actions right now are 1 and 2 so they don't share any bit in
common and the bug has no impact.

No backport is needed.
MEDIUM: checks: make tcp-check perform multiple send() at once

Now instead of seeing many send() calls from multiple "tcp-check send"
rules, we fill the output buffer and try to send all only when we're
not in a send state or when the output buffer is too small for sending
the next message.

This results in a lot less syscalls and avoids filling the network with
many small packets. It will also improve the behaviour of some bogus
servers which expect a complete request in the first packet.