[PATCH 1/3] imap: Restrict COMPRESS to DEFLATE only

[PATCH 1/3] imap: Restrict COMPRESS to DEFLATE only

cmd_compress() and the imap-login proxy side channel looked up any
registered compression handler by name, so a client could request e.g.
COMPRESS ZSTD even though only COMPRESS=DEFLATE is advertised (RFC 4978).

Non-DEFLATE algorithms can require far more memory to decompress (a zstd
decoder alone can hold a 16+ MB window per connection versus ~40 KB for
inflate). This matters now that imap-login handles COMPRESS for proxied
connections: an attacker opening many connections could drive the shared
login process into its vsz_limit and crash it, affecting all clients on
that process.

Only accept DEFLATE on both the backend and the login proxy path.

Gbp-Pq: Name 0001-imap-Restrict-COMPRESS-to-DEFLATE-only.patch