MEDIUM: log: slightly refine the output format of alerts/warnings/etc

MEDIUM: log: slightly refine the output format of alerts/warnings/etc

For about 20 years we've been emitting cryptic messages on warnings and
alerts, that nobody knows how to parse:

  [NOTICE] 126/080118 (3115) : haproxy version is 2.4-dev18-0b7c78-49
  [NOTICE] 126/080118 (3115) : path to executable is ./haproxy
  [WARNING] 126/080119 (3115) : Server default/srv1 is DOWN via static/srv1. 0 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
  [ALERT] 126/080119 (3115) : backend 'default' has no server available!

Hint: the first 3-digit number is the day of year, and the 6 digits
after it represent the time of day in format HHMMSS, then the pid in
parenthesis. These are not quite user-friendly and such cryptic into
are not useful at all.

This patch slightly adjusts the output by performing these minimal changes:
  - removing the date/time, as they were added very early when haproxy
    was meant to be used in foreground as a debugging tool, and they're
    provided in more details in logs nowadays ;

  - better aligning the fields by padding the severity tag to 10 chars.
    The diag output was renamed to "DIAG" only.

Now the output provides this:

  [NOTICE]   (4563) : haproxy version is 2.4-dev18-75a428-51
  [NOTICE]   (4563) : path to executable is ./haproxy
  [WARNING]  (4563) : Server default/srv1 is DOWN via static/srv1. 0 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
  [ALERT]    (4563) : backend 'default' has no server available!

The useless space before the colon was kept so as not to confuse any
possible output parser.

The few entries in the doc referring to this format were adjusted to
reflect the new one.

The change was tagged "MEDIUM" as it may have visible consequences on
home-grown monitoring tools, though it is extremely unlikely due to the
limited extent of these changes.