9ns --mntgen: registry subdirectories are mount points too

A registry entry that is a directory is now served the way the root is:
a synthetic directory listing the real one, dialing the sockets inside
it on walk and recursing into further directories, to max_synth_depth
(8) levels across max_synth_dirs (64) synthetic nodes. That is the
plan9port mntgen shape and the layout zmx now posts under, so a live
session reads at /mnt/9p/zmx/<name>. Before this a directory in the
registry was dialed like a socket and answered EIO for good.

post gains the two entry points the traversal needs: postedDir (the
registry scan, against any directory) and dialPath (a dial by composed
path, no name validation).

Hardening, each from an attack that broke the code:

- BATCH_FORGET carries entries for many owners and puts 0 in the header
  nodeid, so routing it by the header dropped all of them: 32 of 64
  synthetic slots leaked in one close burst and the subdirectories that
  held them answered EIO forever. distributeForgets unpacks the body and
  hands each entry to its owner.
- probe() and connectBlocking() copied a caller's path into the kernel
  address with no bound: a path past sun_path overran the 110-byte stack
  sockaddr (a panic in Debug, silent corruption in ReleaseFast). Both
  refuse it now, probe as `.live` so a claim never deletes what it could
  not inspect.
- That bound then caught 9proc's own listener, which handed probe() the
  whole 108-byte sun_path array instead of the path inside it. The probe
  reads `.live` for anything it cannot ask about, so every stale socket
  became AlreadyListening and no server could ever take a dead
  predecessor's name back. It passes the path now.

Suites: 87/87 root (+7 post/serve attack regressions), 48/48 9ns,
51+88 9ns integration (+4 traversal and slot-recycling checks), 213/0
9ns adversarial, 60/60 9proc plus its adversarial suites with a new
stale-socket takeover check, freestanding green.

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