Initial implementation of MH mailbox format. It is functional in read-only

mode. In read-write, it is able to delete and undelete messages but lacks
`append message' functionality.
Expunge does not delete message files but rather renames them prepending ','
to a filename, after MH fashion.

+/* GNU mailutils - a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Library General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <errno.h>
+
+#include <folder0.h>
+#include <registrar0.h>
+
+static struct _record _mh_record =
+{
+  MU_MH_SCHEME,
+  _url_mh_init, /* Url init.  */
+  _mailbox_mh_init, /* Mailbox init.  */
+  NULL, /* Mailer init.  */
+  _folder_mh_init, /* Folder init.  */
+  NULL, /* back pointer.  */
+  NULL, /* _is_scheme method.  */
+  NULL, /* _get_url method.  */
+  NULL, /* _get_mailbox method.  */
+  NULL, /* _get_mailer method.  */
+  NULL  /* _get_folder method.  */
+};
+record_t mh_record = &_mh_record;
+
+int
+_folder_mh_init (folder_t folder)
+{
+  (void)folder;
+  return 0;
+}

+/* GNU mailutils - a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Library General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <url0.h>
+#include <registrar0.h>
+
+static void
+url_mh_destroy (url_t url)
+{
+  (void) url;
+}
+
+/*
+  MH url
+  mh:path
+*/
+int
+_url_mh_init (url_t url)
+{
+  const char *name = url_to_string (url);
+  size_t len = strlen (name);
+
+  /* reject the obvious */
+  if (name == NULL || strncmp (MU_MH_SCHEME, name, MU_MH_SCHEME_LEN) != 0
+      || len < (MU_MH_SCHEME_LEN + 1) /* (scheme)+1(path)*/)
+    return EINVAL;
+
+  /* do I need to decode url encoding '% hex hex' ? */
+
+  /* TYPE */
+  url->_destroy = url_mh_destroy;
+
+  /* SCHEME */
+  url->scheme = strdup (MU_MH_SCHEME);
+  if (url->scheme == NULL)
+    {
+      url_mh_destroy (url);
+      return ENOMEM;
+    }
+
+  /* PATH */
+  name += MU_MH_SCHEME_LEN; /* pass the scheme */
+  url->path = strdup (name);
+  if (url->path == NULL)
+    {
+      url_mh_destroy (url);
+      return ENOMEM;
+    }
+
+  return 0;
+}