MEDIUM: wdt: always make the faulty thread report its own warnings

MEDIUM: wdt: always make the faulty thread report its own warnings

Warnings remain tricky to deal with, especially for other threads as
they require some inter-thread synchronization that doesn't cope very
well with other parallel activities such as "show threads" for example.

However there is nothing that forces us to handle them this way. The
panic for example is already handled by bouncing the WDT signal to the
faulty thread.

This commit rearranges the WDT handler to make a better used of this
existing signal bouncing feature of the WDT handler so that it's no
longer limited to panics but can also deal with warnings. In order not
to bounce on all wakeups, we only bounce when there is a suspicion,
that is, when the warning timer has been crossed. We'll let the target
thread verify the stuck flag and context switch count by itself to
decide whether or not to panic, warn, or just do nothing and update
the counters.

As a bonus, now all warning traces look the same regardless of the
reporting thread:

   call trace(16):
   |       0x6bc733 <01 00 00 e8 6d e6 de ff]: ha_dump_backtrace+0x73/0x309 > main-0x2570
   |       0x6bd37a <00 00 00 e8 d6 fb ff ff]: ha_thread_dump_fill+0xda/0x104 > ha_thread_dump_one
   |       0x6bd625 <00 00 00 e8 7b fc ff ff]: ha_stuck_warning+0xc5/0x19e > ha_thread_dump_fill
   |       0x7b2b60 <64 8b 3b e8 00 aa f0 ff]: wdt_handler+0x1f0/0x212 > ha_stuck_warning
   | 0x7fd7e2cef3a0 <00 00 00 00 0f 1f 40 00]: libpthread:+0x123a0
   | 0x7ffc6af9e634 <85 a6 00 00 00 0f 01 f9]: linux-vdso:__vdso_gettimeofday+0x34/0x2b0
   |       0x6bad74 <7c 24 10 e8 9c 01 df ff]: sc_conn_io_cb+0x9fa4 > main-0x2400
   |       0x67c457 <89 f2 4c 89 e6 41 ff d0]: main+0x1cf147
   |       0x67d401 <48 89 df e8 8f ed ff ff]: cli_io_handler+0x191/0xb38 > main+0x1cee80
   |       0x6dd605 <40 48 8b 45 60 ff 50 18]: task_process_applet+0x275/0xce9

(cherry picked from commit a06c215f08703b2a2e303b2e23816d4ad09748f0)
Signed-off-by: Willy Tarreau <w@1wt.eu>