ctdb-common: Simplify process registration using linked list

ctdb-common: Simplify process registration using linked list

The way run_proc abstraction is used in run_event, there can be maximum
of 2 processes active at any given time.  So the memory requirements
can be reduced by using a linked list.

New eventd will have multiple run_event instances but will be limited to
3 or 4.  Even then the total number of processes will be less than 10.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>