tevent: added tevent_re_initialise()

tevent: added tevent_re_initialise()

This allows us to re-initialise a tevent context without destroying
the pointer. That means that if someone keeps a long term ptr to the
event context across a fork it will still work.

This also brings the memory handling in single and standard process
models much closer together, which means less bugs that we don't find
with make test.