Supply better and less frequent warnings on circID exhaustion

Fixes the surface behavior of #11553
Switch to random allocation on circuitIDs.

Fixes a possible root cause of 11553 by only making 64 attempts at
most to pick a circuitID.  Previously, we would test every possible
circuit ID until we found one or ran out.

This algorithm succeeds probabilistically. As the comment says:

  This potentially causes us to give up early if our circuit ID
  space is nearly full.  If we have N circuit IDs in use, then we
  will reject a new circuit with probability (N / max_range) ^
  MAX_CIRCID_ATTEMPTS.  This means that in practice, a few percent
  of our circuit ID capacity will go unused.

  The alternative here, though, is to do a linear search over the
  whole circuit ID space every time we extend a circuit, which is
  not so great either.

This makes new vs old clients distinguishable, so we should try to
batch it with other patches that do that, like 11438.
Merge remote-tracking branch 'public/bug11553_024' into bug11553_025

Conflicts:
	src/or/channel.h
Diagnostic warning to see if it's pending destroys causing 11553
Add a rate-limiter for the other circuitID exhaustion warning
Merge remote-tracking branch 'public/bug11553_024' into bug11553_025
scan-build: Add a check for result from getaddrinfo

As documented, getaddrinfo always sets its result when it returns
no error.  But scan-build doesn't know that, and thinks we might
be def
scan-build: check impossible null-pointer case in buffers.c

When maintaining buffer freelists, we don't skip more than there
are, so (*chp) can't be null to begin with.  scan-build has no way
to know that.
scan-build: circuit_cpath_support_ntor had a dead initialization

We were initializing cpath twice, which doesn't make sense.
scan-build: Avoid crashing on BUG in circuit_get_by_rend_token_and_purpose

If we fail in circuit_get_by_rend_token_and_purpose because the
circuit has no rend_info, don't try to reference fiends from its
rend_info when logging an error.  Bugfix on 8b9a2cb68, which is
going into Tor 0.2.5.4-alpha.
scan-build: fix a crash-on-fail possibility in test_policy.c
scan-build: Be consistent with a needless check in circuitmux.c

In circuitmux_detach_all_circuits, we check whether an HT iterator
gives us NULL.  That should be impossible for an HT iterator.  But
our checking it has confused scan-build (justly) into thinking that
our later use of HT_NEXT_RMV might not be kosher.  I'm taking the
coward's route here and strengthening the check.  Bugfix on
fd31dd44. (Not a real bug though)
scan-build: when logging a path length, check build_state.

Throughout circuituse, when we log about a circuit, we log its
desired path length from build_state. scan-build is irrationally
concerned that build_state might be NULL.
scan-build: get_proxy_addrport should always set its outputs

When get_proxy_addrport returned PROXY_NONE, it would leave
addr/port unset. This is inconsistent, and could (if we used the
function in a stupid way) lead to undefined behavior. Bugfix on
5b050a9b0, though I don't think it affects tor-as-it-is.
scan-build: in cpuworker, initialize tv_start

scan-build doesn't realize that a request can't be timed at the end
unless it's timed at the start, and so it's not possible for us to
be subtracting start from end without start being set.
Nevertheless, let's not confuse it.
scan-build: Have clear_pending_onions walk the lists more obviously

As it stands, it relies on the fact that onion_queue_entry_remove
will magically remove each onionskin from the right list.  This
patch changes the logic to be more resilient to possible bugs in
onion_queue_entry_remove, and less confusing to static analysis tools.
scan-build: bulletproof last-chance errormsg generation in rendservice.c

If 'intro' is NULL in these functions, I'm pretty sure that the
error message must be set before we hit the end.  But scan-build
doesn't notice that, and is worried that we'll do a null-pointer
dereference in the last-chance errormsg generation.
scan-build: limit hashtable size so it always fits in SSIZE_MAX

scan-build recognizes that in theory there could be a numeric overflow
here.

This can't numeric overflow can't trigger IRL, since in order to fill a
hash table with more than P=402653189 buckets with a reasonable load
factor of 0.5, we'd first have P/2 malloced objects to put in it--- and
each of those would have to take take at least sizeof(void*) worth of
malloc overhead plus sizeof(void*) content, which would run you out of
address space anyway on a 32-bit system.
scan-build: truncate tinytest hexified outputs to 1024 bytes.

scan-build didn't like the unlimited version since we might need to
overflow size_t to hexify a string that took up half our address
space. (!)
scan-build: close stdio FILEs on error in tor-gencert

This is harmless, since tor-gencert exits right afterwards, but it's
best to clean up after ourselves.