[Add] Auth ldap async

[farvour]

Dirty implementation of using POSIX threads to invoke the deferred authentication plugin call for the OpenVPN LDAP Auth plugin.

Issue: When a connection is initiated to OpenVPN, the authentication runs on the main thread, which means slow LDAP servers, LDAP proxies (e.g. using DUO 2Factor Auth etc) that hang or wait will block all traffic in and out of the VPN.

Solution: Use a thread and defer the call taking advantage of OpenVPN 2.3.14+'s v2 implementation hook and write to the acf file.

Caveats: I didn't work to make this backward compatible with older Objective-C (non-modern) runtime, so it requires CentOS 7+ (gcc 4.6 and up?). If someone else wants to add all of the conditionals / changes to do that please feel free!

Disclaimer: My ObjC and C strengths are not very strong, and I made the best what I could with the solution. It solves the problem in our application of it and that is what I was shooting for.

[k0ste]

@farvour can you rebase your PR? Current master is support modern gcc's.

[k0ste]

Resolve #66

[odnanref]

Any news on this?

[kirik]

After 2 weeks of debugging and tries to find a root cause of packet loss/high latency when using openvpn I came to this issue. I use "slow" LDAP server which answers in 2-3 seconds. At this time all other traffic was just waiting while single user completes his/her authentication process... Any workarounds for this so far? Or having fast LDAP server is the only option for now? Thanks