BUG/MINOR: checks: don't consider errno and use conn->err_code

The last fix on checks (02b0f58: BUG/MEDIUM: checks: fix a long-standing
issue with reporting connection errors) tried to isolate error codes
retrieved from the socket in order to report appropriate messages. The
only thing is that we must not pre-initialize err to errno since we're
not in I/O context anymore and errno will be the one of the last syscall
(whatever it was). However we can complete the message with more info
from the transport layer (eg: SSL can inform us we were in a handshake).

Also add a catch-all case for CO_FL_ERROR when the connection was
established. No check currently seem to leave this case open, but better
catch it because it's hard to find all possible cases.

Error handling in checks is complex because some stuff must be done in
the central task (mandatory at least for timeouts) and other stuff is
done closer to the data.

Since checks have their own buffers now, we could move everything to
the main task and only keep the low-level I/O for sending/retrieving
data to/from this buffer. It would also avoid sending logs from the
I/O context!
BUG/MEDIUM: checks: also update the DRAIN state from the web interface

In commit 8c3d0be (MEDIUM: Add DRAIN state and report it on the stats page),
the drain state was updated on every weight change except those that can be
sent via the web interface. This caused inconsistent state combinations to
be reported in the stats depending on the sequence (web then cli vs cli
then web).

It would seem that a call to set_server_drain_state() from within
server_recalc_eweight() would simplify things but that's not completely
certain yet.
MINOR: stats: remove some confusion between the DRAIN state and NOLB

We now have to report 2 conflicting information on the stats page :
  - NOLB  = server which returns 404 and stops load balancing ;
  - DRAIN = server with a weight forced to zero

The DRAIN state was previously detected from eweight==0 and represented in
blue so that a temporarily disabled server was noticed. This was done by
commit cc8bb92 (MINOR: stats: show soft-stopped servers in different color).
This choice suffered from a small defect however, which is that a server
with a zero weight was reported in this color whatever its state (even down
or switching).

Also, one of the motivations for the color above was because the NOLB state
is barely detectable as it's very close to the UP state.

Since commit 8c3d0be (MEDIUM: Add DRAIN state and report it on the stats page),
we have the new DRAIN state to show servers with a zero weight. The colors are
unfortunately very close to those of the MAINT state, and some users were
confused by the disappearance of the blue bars.

Additionally, the NOLB state had precedence over DRAIN, which could be an
issue since DRAIN is the only thing the admin can act on, so once NOLB was
shown, there was nothing to indicate that the weight was forced to zero.
By switching the two priorities we can report DRAIN (forced mode) before
NOLB (detected mode).

The best solution to fix all this is to reuse the previous blue color for
all cases where weight == 0, whether it's set by config / agent / cli (DRAIN)
or detected by a 404 response (NOLB). However we only use this color when the
server is 100% UP. If it's going down we switch to the usual yellow color
showing failed checks, and when it's down it keeps its usual red color.

That way, a blue bar on the display indicates a server not taking new
sessions but perfectly up. And other colors keep their usual meaning.