Use pthread_rwlock for RW locks in userland

[paullouisageneau]

This PR introduces pthread_rwlock instead of pthread_mutex to implement RW locks, as suggested in #538.

[tuexen]

You changed the

• inp info lock
• inp lock
• ipi addr lock

In FreeBSD we have some test coverage with inp info lock and the ipi addr lock being r/w locks. But we haven't used r/w locks for inp lock and I don't think you would gain much. So I would prefer not to change the inp lock yet. Could you revert that part?

A different question: Do you know how to change the inp info lock and the ipi addr lock on Windows? It would be great to have the same locking on all platforms supported by usrsctp...

[paullouisageneau]

In FreeBSD we have some test coverage with inp info lock and the ipi addr lock being r/w locks. But we haven't used r/w locks for inp lock and I don't think you would gain much. So I would prefer not to change the inp lock yet. Could you revert that part?

A different question: Do you know how to change the inp info lock and the ipi addr lock on Windows? It would be great to have the same locking on all platforms supported by usrsctp...

Sure, I reverted the inp lock and changed the locks also for Windows. There was no shared locks before Vista (yeah...) so I excluded WIndows XP from the change.

[tuexen]

Thanks for providing the patch!