qa: Fix `interface_ipc*.py` on NetBSD and prevent future regressions #36587

Easy to review using the following:
```
git diff --color-moved=dimmed-zebra --color-moved-ws=allow-indentation-change
```

Since `-ipcbind` reserves one listening socket plus
`ipc::DEFAULT_MAX_CONNECTIONS` (16) client slots in `min_required_fds`,
the framework's `-maxconnections=76` no longer fits the 256 file
descriptor budget on NetBSD, and every `interface_ipc*.py` node starts
with "Warning: Reducing -maxconnections ...", which the framework treats
as a failure.

Lower `-maxconnections` by that reservation on IPC-enabled nodes only,
so the rest of the suite keeps its P2P connection slots.

On select()-based platforms such as NetBSD, `bitcoind` can use at most
`FD_SETSIZE` (256) file descriptors, so the functional test framework
pins `-maxconnections` to that budget. Any change that grows
`min_required_fds` then makes every node print "Warning: Reducing
-maxconnections ...", which the framework treats as a failure. Such
regressions are currently caught only by nightly NetBSD CI (see #31537,

Add a `CI_LIMIT_FD_COUNT` knob that runs functional test node processes
through a wrapper capping `RLIMIT_NOFILE`, and enable it for the macOS
native job with NetBSD's value of 256.