MINOR: fd: add a generation number to file descriptors

MINOR: fd: add a generation number to file descriptors

This patch adds a counter of close() on file descriptors in the fdtab.
The goal is to better detect if reported events concern the current or
a previous file descriptor. For now the counter is only added, and is
showed in "show fd" as "gen". We're reusing unused space at the end of
the struct. If it's needed for something more important later, this
patch can be reverted.

(cherry picked from commit d155924efe453faa0ea5b5e62e6a3e2bf7ee7358)
Signed-off-by: Willy Tarreau <w@1wt.eu>