Stream support rewritten from scratch.

Does not compile yet.

New files:
* mailbox/streamref.c
* include/mailutils/sys/streamref.h
* include/mailutils/sys/file_stream.h
* include/mailutils/sys/header_stream.h
* include/mailutils/sys/mapfile_stream.h
* include/mailutils/sys/memory_stream.h
* include/mailutils/sys/message_stream.h
* include/mailutils/sys/prog_stream.h
* include/mailutils/sys/socket_stream.h
* include/mailutils/sys/stdio_stream.h
* include/mailutils/sys/stream.h
* include/mailutils/sys/streamtrans.h
* mailbox/prog_stream.c
* mailbox/stdio_stream.c
* mailbox/stream_printf.c
* mailbox/stream_vprintf.c
* mailbox/temp_file_stream.c
* mailbox/streamcpy.c
* mailbox/binflt.c
* mailbox/crlfflt.c
* mailbox/fltstream.c
* mailbox/qpflt.c
* mailbox/linelenflt.c

Removed files:
* mailbox/filter_rfc822.c
* mailbox/filter_trans.c

Moved files (with edits):
* libproto/include/filter0.h -> include/mailutils/sys/filter.h
* libproto/include/header0.h -> include/mailutils/sys/header.h
* libproto/include/message0.h -> include/mailutils/sys/message.h
* libproto/include/mime0.h -> include/mailutils/sys/mime.h

Modified files:
* comsat/action.c
* config/mailutils-config.c
* examples/base64.c
* examples/cpp/msg-send.cc
* examples/header.c
* examples/http.c
* examples/iconv.c
* examples/mimetest.c
* examples/msg-send.c
* examples/mta.c
* examples/murun.c
* examples/nntpclient.c
* examples/pop3client.c
* imap4d/append.c
* imap4d/auth_gsasl.c
* imap4d/fetch.c
* imap4d/preauth.c
* imap4d/search.c
* imap4d/util.c
* include/mailutils/body.h
* include/mailutils/folder.h
* include/mailutils/header.h
* include/mailutils/mailbox.h
* include/mailutils/mailer.h
* include/mailutils/mailutils.h
* include/mailutils/message.h
* include/mailutils/stream.h
* include/mailutils/sys/Makefile.am
* include/mailutils/sys/filter.h
* include/mailutils/sys/header.h
* include/mailutils/sys/message.h
* include/mailutils/sys/mime.h
* include/mailutils/sys/nntp.h
* include/mailutils/sys/pop3.h
* include/mailutils/sys/stream.h
* include/mailutils/types.hin
* lib/mailcap.c
* lib/mu_dbm.h
* libmu_argp/muinit.c
* libmu_auth/ldap.c
* libmu_auth/tls.c
* libmu_scm/Makefile.am
* libmu_scm/mu_body.c
* libmu_scm/mu_message.c
* libmu_scm/mu_port.c
* libmu_sieve/actions.c
* libmu_sieve/extensions/pipe.c
* libmu_sieve/extensions/spamd.c
* libmu_sieve/extensions/vacation.c
* libproto/mailer/smtp.c
* libproto/mbox/mboxscan.c
* libproto/mbox/mbox.c
* libproto/mbox/mbox0.h
* libproto/nntp/nntp_article.c
* libproto/nntp/nntp_carrier.c
* libproto/nntp/nntp_destroy.c
* libproto/nntp/nntp_ihave.c
* libproto/nntp/nntp_post.c
* libproto/nntp/nntp_readline.c
* libproto/nntp/nntp_sendline.c
* libproto/pop/pop3_carrier.c
* libproto/pop/pop3_destroy.c
* libproto/pop/pop3_readline.c
* libproto/pop/pop3_sendline.c
* libproto/pop/pop3_stat.c
* maidag/mailtmp.c
* mail/decode.c
* mail/escape.c
* mail/msgset.y
* mail/pipe.c
* mail/print.c
* mail/send.c
* mail/top.c
* mail/util.c
* mail/write.c
* mailbox/Makefile.am
* mailbox/amd.c
* mailbox/attachment.c
* mailbox/body.c
* mailbox/cfg_driver.c
* mailbox/cfg_format.c
* mailbox/debug.c
* mailbox/file_stream.c
* mailbox/folder.c
* mailbox/hdritr.c
* mailbox/header.c
* mailbox/mailbox.c
* mailbox/mailcap.c
* mailbox/mailer.c
* mailbox/mapfile_stream.c
* mailbox/memory_stream.c
* mailbox/message.c
* mailbox/message_stream.c
* mailbox/mime.c
* mailbox/mimehdr.c
* mailbox/mutil.c
* mailbox/prog_stream.c
* mailbox/progmailer.c
* mailbox/rfc2047.c
* mailbox/socket_stream.c
* mailbox/stream.c
* mailbox/stream_vprintf.c
* mailbox/tcp.c
* mailbox/vartab.c
* mailbox/wicket.c
* mh/burst.c
* mh/comp.c
* mh/compcommon.c
* mh/forw.c
* mh/mh_ctx.c
* mh/mh_format.c
* mh/mh_init.c
* mh/mh_list.c
* mh/mh_whatnow.c
* mh/mhl.c
* mh/mhn.c
* mh/pick.y
* mh/repl.c
* mimeview/mimeview.c
* mu-aux/generr.awk
* pop3d/extra.c
* pop3d/pop3d.h
* pop3d/retr.c
* pop3d/top.c
* readmsg/msglist.c
* readmsg/readmsg.c
* sieve/sieve.c