merge: upstream adoption of compress

 * Compress patch from the neomutt repository.
 * Compress: prefix external functions with "mutt_"
 * Compress: remove buffy stats "improvisation".
 * Compress: fix several logic and memory bugs.
 * Compress: add delegate calls to open_append and close mx_ops functions.
 * Compress: safe_fopen() the tempfile, to prevent tempfile attacks.
 * Compress: pull the lock/unlock operations into the open,close,sync operations.
 * Create mx_ops.sync operation. Refactor compress to use the mx_ops.sync.
 * Compress: fix check_mailbox and sync_mailbox.
 * Compress: escape single quotes when invoking the compress/decompress commands.
 * Compress: check mailbox type if possible when appending.
 * Ensure the compressed documentation is always built.
 * Improve two compress translation messages.
 * Revert "Compress: remove buffy stats "improvisation"."