MEDIUM: mworker: set the iocb of the socketpair without using fd_insert()

MEDIUM: mworker: set the iocb of the socketpair without using fd_insert()

The worker was previously changing the iocb of the socketpair in the
worker by mworker_accept_wrapper(). However, it was done using
fd_insert() instead of changing directly the callback in the
fdtab[].iocb pointer.

This patch cleans up this by part by removing fd_insert().

It also stops setting tid_bit on the thread mask, the socketpair will be
handled by any thread from now.