Implement a reload failure counter which counts the number of failure
since the last success. This counter is available in 'show proc' over
the master CLI.

The relative_pid is always 1. In mworker mode we also have a
child->relative_pid which is always equalt relative_pid, except for a
master (0) or external process (-1), but these types are usually tested
for, except for one place that was amended to carefully check for the
PROC_O_TYPE_WORKER option.

Changes were pretty limited as most usages of relative_pid were for
designating a process in stats output and peers protocol.

No change was necessary. Include from wdt.c was dropped since unneeded.

One function prototype makes reference to struct mworker_proc which was
not defined there but in global.h instead. This definition, along with
the PROC_O_* fields were moved to mworker-t.h instead.