CI only for yas-run/yas#88 (keep output head and tail, on #86) — do not merge - #42

…run#86)

* Report a spawned process's exit unasked: bash in one round trip

A SPAWN with the new opt-in flag REPORT_EXIT (16) gets its process's exit as
an EXIT Event (Process 0x0002, sensitive) once it is final: the record a WAIT
would return then. A command's SPAWN Result, output and exit thus all come
from one request, where a client had to send WAIT and wait a round trip more.

- Servers advertise it in a new optional family limit, LAUNCHER_FLAGS_EXTENDED
  (tag 19), since clients from before accept at most 12 in tag 11; a client sets
  REPORT_EXIT only when tag 19 offers it, so either side may be older.
- The report takes none of the session's pending WAITs, is sent once (a SPAWN
  retried under its operation ID shares it), and outlives the streams: when the
  spawning session's attachment goes before the exit (its streams dropped, their
  route failed), the server waits for the exit itself and still sends it.
- The streams go on at the pace of their credit, so the EXIT may arrive before
  their last bytes and CLOSE.
- yas-client sets the flag whenever offered; Process::wait/wait_timeout then
  take no request and keep the reported exit. Attached processes, and servers
  without the flag, still WAIT.

Tests (client_host): a_spawned_process_reports_its_exit_without_a_wait (with
the only WAIT held, a non-zero exit with stderr, output beyond the stream
buffer read while the exit is awaited, a signal, and another session's
attachment that still WAITs), and a_server_from_before_report_exit_is_waited_for
(ignored; YAS_OLD_SERVER=<yas without the flag>).

* Report an exit only from the attachment that saw it; WAIT once that goes

Review of yas-run#86: the server's fallback wait, for a REPORT_EXIT process whose
attachment went before its exit, bound the process again (a detached one's
slot was taken back and ATTACH answered CONFLICT), hung Process::wait when
that wait failed, and an attachment aborted mid-send lost the EXIT.

The fallback is gone: the attachment reports the exit, from a task of its
own so removing the attachment meanwhile cannot cut the EXIT short. When the
attachment goes first (a Transfer RESET on any of its streams, stdin
included, sent or received, or a DETACH) no EXIT comes, and yas-client
knows: its router marks the process's report lost, and Process::wait and
wait_timeout then WAIT, after taking an EXIT that already arrived.

* Make tag 19 a set of SPAWN flags whose unknown bits clients ignore

LAUNCHER_FLAGS_EXTENDED's hard maximum was 28, the flags it carries
today: a later server adding one would have failed the HELLO of every
client from this PR, as tag 11's maximum of 12 would have failed older
clients for REPORT_EXIT. Its hard maximum is now 65535 (any u16 of SPAWN
flags) and Limits::from_extensions keeps only the flags it knows, so the
next flag needs no new tag.