Add a bunch of files to .gitignore
Add fudge factor in subscription sharding

Header lengths are not constant-size because Zcode escapes bytes 0xFF and 0x00
into two bytes. If we end up filling up close to all the space we have and
Z_SendFragmentedNotice then computes a header length larger than ours by
enough, the message gets fragmented.

Getting it fragmented is especially unfortunate because only the first of a
fragmented notice ever has a SERVACK survive. (They all get SERVACKs, but
libzephyr kindly drops all but the first on the floor.)

This isn't a watertight fix; we may get really really unlucky and blow up 13
bytes in the authenticator and checksum. But that's not likely, and a proper
fix would involve either computing based on the maximum possible authenticator
size (wasteful and hard to bound tightly) or changing to protocol to use a less
inappropriate encoding.
Defensively avoid waiting on non-initial SERVACKs

Although the previous commit should make it very unlikely we screw up the
subscription sharding, be defensive about waiting for SERVACKs. ZSubscribeTo
does mess up, Z_SendFragmentedNotice will shard with a z_multiuid. In that
case, although the second packet will get a SERVACK, Z_ReadWait kindly drops it
on the floor. The ZIfNotice will then just hang.

Tested by bumping zwgc's BATCH_SIZE up to 200, reverting the previous commit,
and strace.