Use scheduler locks instead of critical sections and mutex

In the current implementation the IP stack uses interrupt locks and mutexes in some frequently used locations. Interrupt locks worsen the realtime performance and are unnecessary since LwIP is never accessed from interrupt context. Mutexes require some performance and should be avoided in case of a small number of code lines.

The above locking mechanisms are to be replaced by scheduler locks. Especially for ecos operating system this results in a performance boost but also improves the situation for rcX.