imap: add support for COMPRESS=DEFLATE, RFC 4978

- added mutt_zstrm which allows wrapping an existing connection with
  deflate/inflate (zlib compression)
- call mutt_zstrm_wrap_conn when setting up an IMAP connection if the
  server supports COMPRESSION=DEFLATE and imap_deflate evaluates to yes
- add config quad-option imap_deflate enable/disable use of
  (de)compression for IMAP connections, defaulting to yes
- add configure check for zlib, --with-zlib to detect if mutt_zstrm can
  (or should) be built

Tested against a Dovecot IMAP server, observed easily 7x compression
rates on received data, and 5x on sent data for a normal session.  Rates
can be observed when the connection is closed on debug level 4 and
higher.

Bug: https://gitlab.com/muttmua/mutt/issues/92

1 files changed, 6 insertions, 0 deletions

diff --git a/mutt.h b/mutt.h
index 41bec8cc..aa2ab672 100644
--- a/mutt.h
+++ b/mutt.h
@@ -318,6 +318,9 @@ enum
   OPT_FORWEDIT,
   OPT_FCCATTACH,
   OPT_INCLUDE,
+#if defined(USE_IMAP) && defined(USE_ZLIB)
+  OPT_IMAPDEFLATE,
+#endif
   OPT_MFUPTO,
   OPT_MIMEFWD,
   OPT_MIMEFWDREST,
@@ -444,6 +447,9 @@ enum
   OPTIMAPPEEK,
   OPTIMAPQRESYNC,
   OPTIMAPSERVERNOISE,
+#ifdef USE_ZLIB
+  OPTIMAPDEFLATE,
+#endif
 #endif
 #if defined(USE_SSL)
 # ifndef USE_SSL_GNUTLS