xshared: Implement xtables lock timeout using signals

Previously, if a lock timeout is specified using `-wN `, flock() is
called using LOCK_NB in a loop with a sleep. This results in two issues.

The first issue is that the process may wait longer than necessary when
the lock becomes available. For this the `-W` option was added, but this
requires fine-tuning.

The second issue is that if lock contention is high, invocations using
`-w` (without a timeout) will always win lock acquisition from
invocations that use `-w N`. This is because invocations using `-w` are
actively waiting on the lock whereas those using `-w N` only check from
time to time whether the lock is free, which will never be the case.

This patch removes the sleep loop and deprecates the `-W` option (making
it non-functional). Instead, flock() is always called in a blocking
fashion, but the alarm() function is used with a non-SA_RESTART signal
handler to cancel the system call.

Signed-off-by: Jethro Beekman <jethro@fortanix.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

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