DEBUG: pools: apply poisonning on every allocated pool

Till now, when memory poisonning was enabled, it used to be done only
after a calloc(). But sometimes it's not enough to detect unexpected
sharing, so let's ensure that we now poison every allocation once it's
in place. Note that enabling poisonning significantly hurts performance
(it can typically half the overall performance).
MINOR: samples: add the word converter.

word(<index>,<delimiters>)
  Extracts the nth word considering given delimiters from an input string.
  Indexes start at 1 and delimiters are a string formatted list of chars.
BUG/MAJOR: sessions: unlink session from list on out of memory

Since embryonic sessions were introduced in 1.5-dev12 with commit
2542b53 ("MAJOR: session: introduce embryonic sessions"), a major
bug remained present. If haproxy cannot allocate memory during
session_complete() (for example, no more buffers), it will not
unlink the new session from the sessions list. This will cause
memory corruptions if the memory area from the session is reused
for anything else, and may also cause bogus output on "show sess"
on the CLI.

This fix must be backported to 1.5.