mem-pool: Fix potential memory leak and lost leases when reassigning leases

If no offline leases are available for the current client and assigning online
leases is disabled, and if all IPs of the pool have already been assigned to
clients we look for offline leases that previously were assigned to other
clients.

In case the current client has online leases the previous code would
replace the existing mapping entry and besides resulting in a memory leak
the online leases would be lost forever (even if the client later releases
the addresses).  If this happens repeatedly the number of available addresses
would decrease even though the total number of online and offline leases seen
in `ipsec leases` would indicate that there are free addresses available.

Fixes #764.
kernel-netlink: Add a stub for a test-runner
kernel-netlink: Add a simple send message test querying available links
kernel-netlink: Support parallel Netlink queries

Instead of locking the socket exclusively to wait for replies, use watcher
to wait for and read in responses asynchronously. This allows multiple parallel
Netlink queries, which can significantly improve performance if the kernel
Netlink layer has longer latencies and supports parallel queries.

For vanilla Linux, parallel queries don't make much sense, as it usually returns
EBUSY for the relevant dump requests. This requires a retry, and in the end
makes queries more expensive under high load.

Instead of checking the Netlink message sequence number to detect multi-part
messages, this code now relies on the NLM_F_MULTI flag to detect them. This
has previously been avoided (by 1d51abb7). It is unclear if the flag did not
work correctly on very old Linux kernels, or if the flag was not used
appropriately by strongSwan. The flag seems to work just fine back to 2.6.18,
which is a kernel still in use by RedHat/CentOS 5.
kernel-netlink: Retry netlink query while kernel returns EBUSY

If the kernel can't execute a Netlink query because a different query is already
active, it returns EBUSY. As this can happen now as we support parallel queries,
retry on this error condition.
kernel-netlink: Add a stress test with several threads doing Netlink exchanges
kernel-netlink: Implement configurable Netlink request retransmission
kernel-netlink: Add a compile-time hook to simulate request message loss
kernel-netlink: Add test cases for successful and timing out retransmissions
kernel-netlink: Add non-dumping variants of echo and stress tests
kernel-netlink: Release lock while doing Netlink NEW/DELADDR operations

Besides that it can improve throughput, it avoids a deadlock situation. If
all threads are busy, watcher will invoke the FD notification for NEWADDR
events itself. If the lock is held, it gets locked up. As watcher is not
dispatching anymore, it can't signal Netlink socket send() completion, and
the send() operation does not return and keeps the lock.
kernel-netlink: Add options to enable parallel Netlink queries explicitly

As under vanilla Linux the kernel can't handle parallel dump queries and returns
EBUSY, it makes not much sense to use them. Disable parallel queries by default
to basically restore original behavior, improving performance.
conf: Document kernel-netlink retransmission and parallelization options
kernel-netlink: Alternatively support global port based IKE bypass policies

The socket based IKE bypass policies are usually superior, but not supported
on all networking stacks. The port based variant uses global policies for the
UDP ports we have IKE sockets for.
kernel-netlink: Fallback to UDP if detecting socket protocol fails

getsockopt(SO_PROTOCOL) is not supported before 2.6.32. Default to UDP if
either the SO_PROTOCOL define is missing or the syscall fails.
kernel-netlink: Add an option to enforce using XFRM_MSG_UPDPOLICY
kernel-netlink: Optionally ignore errors resulting from response message loss

As some backends over unreliable transport do not cache response messages,
retransmissions due the loss of responses perform the operation again. Add an
option to ignore some errors arising from such duplicate operations.

Note: This approach can't distinguish between real EXIST/NOTFOUND errors
and packet failures, and therefore is a source of race conditions and can't
detect any of these errors actually happening. Therefore that behavior is
disabled by default, and can be enabled with the ignore_retransmit_errors
strongswan.conf option.

To properly distinguish between real and retransmission errors, a Netlink
backend should implement retransmission detection using sequence numbers.
Merge branch 'netlink-extensions'

Introduces options to enable concurrent Netlink queries. While this does not
make much sense on vanilla Linux, this can help on third party stacks to
increase throughput if longer latencies are to expect. Netlink message
retransmission can be optionally enabled if transmission is unreliable.
Non-socket based IKE bypass policies and other tweaks bring better compatibility
to third party stacks using Netlink.
windows: Provide a poll(2) wrapper calling WSAPoll()
libradius: Use poll(2) to wait for RADIUS responses