Kill p_instance from proc.c and remove static proc_id unused variables.

Kill p_instance from proc.c and remove static proc_id unused variables.

To keep the debug functionality intact and correct we'll use the pid
field in the imsg header to pass the instance number. Remember to always
pass 'ps_instance + 1' otherwise libutil will fill imsg header pid field
with the imsgbuf pid (which is the current process pid).

ok reyk@