[HACKERS] The error message "sorry, too many clients already" is imprecise

I recently started getting the "sorry, too many clients already" error.
There are currently four places that can generate it, but fortunately
log_error_verbosity was set to verbose so I was able to see that in this
case the warning was generated by proc.c:InitProcess().

But it's still not much, because there are three different lists you can
run out of and get the same error message from InitProcess(). I was
lucky to be able to rule out two of them. max_connections is set to much
more than the sum of possible connections from all relevant pgBouncer
instances we have, so it's not hitting max_connections. Also, this is on
9.4 and we don't use any funny extensions, so it's probably not running
out of bgworkerFreeProcs either. autovacFreeProcs is what's left but
this is still just a guess and I'd prefer to actually know.

I've made a hack for myself (attached diff against 9.4) which adds a
DETAIL-level message telling me which proc list was saturated. It's not
committable in its current form because of a C99 feature and perhaps for
other reasons.

By the way, I've also noticed that the InitProcess() can segfault upon
hitting set_spins_per_delay(procglobal->spins_per_delay). This only
happens when I run REL9_4_STABLE under gdb, set a breakpoint on
InitProcess, see an autovacuum launcher hit the breakpoint and tell gdb
to continue. "p procglobal->spins_per_delay" says "Cannot access memory
at address 0xf01b2f90". Maybe this means nothing.

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